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222 KiB
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From owner-pgsql-hackers@hub.org Sun Jun 14 18:45:04 1998
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Received: from hub.org (hub.org [209.47.148.200])
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id AA07922; Sun, 14 Jun 1998 15:35:13 -0700
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From: dg@illustra.com (David Gould)
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Message-Id: <9806142235.AA07922@hawk.illustra.com>
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Subject: [HACKERS] performance tests, initial results
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To: pgsql-hackers@postgreSQL.org
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Date: Sun, 14 Jun 1998 15:35:13 -0700 (PDT)
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Mime-Version: 1.0
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Content-Type: text/plain; charset=US-ASCII
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Sender: owner-pgsql-hackers@hub.org
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Precedence: bulk
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Status: RO
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I have been playing a little with the performance tests found in
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pgsql/src/tests/performance and have a few observations that might be of
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minor interest.
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The tests themselves are simple enough although the result parsing in the
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driver did not work on Linux. I am enclosing a patch below to fix this. I
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think it will also work better on the other systems.
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A summary of results from my testing are below. Details are at the bottom
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of this message.
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My test system is 'leslie':
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linux 2.0.32, gcc version 2.7.2.3
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P133, HX chipset, 512K L2, 32MB mem
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NCR810 fast scsi, Quantum Atlas 2GB drive (7200 rpm).
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Results Summary (times in seconds)
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Single txn 8K txn Create 8K idx 8K random Simple
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Case Description 8K insert 8K insert Index Insert Scans Orderby
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=================== ========== ========= ====== ====== ========= =======
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1 From Distribution
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P90 FreeBsd -B256 39.56 1190.98 3.69 46.65 65.49 2.27
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IDE
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2 Running on leslie
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P133 Linux 2.0.32 15.48 326.75 2.99 20.69 35.81 1.68
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SCSI 32M
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3 leslie, -o -F
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no forced writes 15.90 24.98 2.63 20.46 36.43 1.69
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4 leslie, -o -F
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no ASSERTS 14.92 23.23 1.38 18.67 33.79 1.58
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5 leslie, -o -F -B2048
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more buffers 21.31 42.28 2.65 25.74 42.26 1.72
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6 leslie, -o -F -B2048
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more bufs, no ASSERT 20.52 39.79 1.40 24.77 39.51 1.55
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Case to Case Difference Factors (+ is faster)
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Single txn 8K txn Create 8K idx 8K random Simple
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Case Description 8K insert 8K insert Index Insert Scans Orderby
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=================== ========== ========= ====== ====== ========= =======
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leslie vs BSD P90. 2.56 3.65 1.23 2.25 1.83 1.35
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(noflush -F) vs no -F -1.03 13.08 1.14 1.01 -1.02 1.00
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No Assert vs Assert 1.05 1.07 1.90 1.06 1.07 1.09
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-B256 vs -B2048 1.34 1.69 1.01 1.26 1.16 1.02
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Observations:
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- leslie (P133 linux) appears to be about 1.8 times faster than the
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P90 BSD system used for the test result distributed with the source, not
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counting the 8K txn insert case which was completely disk bound.
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- SCSI disks make a big (factor of 3.6) difference. During this test the
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disk was hammering and cpu utilization was < 10%.
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- Assertion checking seems to cost about 7% except for create index where
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it costs 90%
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- the -F option to avoid flushing buffers has tremendous effect if there are
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many very small transactions. Or, another way, flushing at the end of the
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transaction is a major disaster for performance.
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- Something is very wrong with our buffer cache implementation. Going from
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256 buffers to 2048 buffers costs an average of 25%. In the 8K txn case
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it costs about 70%. I see looking at the code and profiling that in the 8K
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txn case this is in BufferSync() which examines all the buffers at commit
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time. I don't quite understand why it is so costly for the single 8K row
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txn (35%) though.
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It would be nice to have some more tests. Maybe the Wisconsin stuff will
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be useful.
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----------------- patch to test harness. apply from pgsql ------------
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*** src/test/performance/runtests.pl.orig Sun Jun 14 11:34:04 1998
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Differences %
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----------------- patch to test harness. apply from pgsql ------------
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*** src/test/performance/runtests.pl.orig Sun Jun 14 11:34:04 1998
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--- src/test/performance/runtests.pl Sun Jun 14 12:07:30 1998
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***************
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*** 84,123 ****
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open (STDERR, ">$TmpFile") or die;
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select (STDERR); $| = 1;
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! for ($i = 0; $i <= $#perftests; $i++)
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! {
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$test = $perftests[$i];
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($test, $XACTBLOCK) = split (/ /, $test);
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$runtest = $test;
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! if ( $test =~ /\.ntm/ )
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! {
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! #
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# No timing for this queries
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- #
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close (STDERR); # close $TmpFile
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open (STDERR, ">/dev/null") or die;
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$runtest =~ s/\.ntm//;
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}
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! else
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! {
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close (STDOUT);
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open(STDOUT, ">&SAVEOUT");
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print STDOUT "\nRunning: $perftests[$i+1] ...";
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close (STDOUT);
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open (STDOUT, ">/dev/null") or die;
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select (STDERR); $| = 1;
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! printf "$perftests[$i+1]: ";
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}
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do "sqls/$runtest";
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# Restore STDERR to $TmpFile
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! if ( $test =~ /\.ntm/ )
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! {
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close (STDERR);
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open (STDERR, ">>$TmpFile") or die;
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}
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-
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select (STDERR); $| = 1;
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$i++;
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}
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--- 84,116 ----
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open (STDERR, ">$TmpFile") or die;
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select (STDERR); $| = 1;
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! for ($i = 0; $i <= $#perftests; $i++) {
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$test = $perftests[$i];
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($test, $XACTBLOCK) = split (/ /, $test);
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$runtest = $test;
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! if ( $test =~ /\.ntm/ ) {
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# No timing for this queries
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close (STDERR); # close $TmpFile
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open (STDERR, ">/dev/null") or die;
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$runtest =~ s/\.ntm//;
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}
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! else {
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close (STDOUT);
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open(STDOUT, ">&SAVEOUT");
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print STDOUT "\nRunning: $perftests[$i+1] ...";
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close (STDOUT);
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open (STDOUT, ">/dev/null") or die;
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select (STDERR); $| = 1;
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! print "$perftests[$i+1]: ";
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}
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do "sqls/$runtest";
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# Restore STDERR to $TmpFile
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! if ( $test =~ /\.ntm/ ) {
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close (STDERR);
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open (STDERR, ">>$TmpFile") or die;
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}
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select (STDERR); $| = 1;
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$i++;
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}
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***************
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*** 128,138 ****
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open (TMPF, "<$TmpFile") or die;
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open (RESF, ">$ResFile") or die;
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! while (<TMPF>)
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! {
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! $str = $_;
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! ($test, $rtime) = split (/:/, $str);
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! ($tmp, $rtime, $rest) = split (/[ ]+/, $rtime);
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! print RESF "$test: $rtime\n";
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}
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--- 121,130 ----
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open (TMPF, "<$TmpFile") or die;
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open (RESF, ">$ResFile") or die;
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! while (<TMPF>) {
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! if (m/^(.*: ).* ([0-9:.]+) *elapsed/) {
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! ($test, $rtime) = ($1, $2);
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! print RESF $test, $rtime, "\n";
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! }
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}
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------------------------------------------------------------------------
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------------------------- testcase detail --------------------------
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1. from distribution
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DBMS: PostgreSQL 6.2b10
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OS: FreeBSD 2.1.5-RELEASE
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HardWare: i586/90, 24M RAM, IDE
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StartUp: postmaster -B 256 '-o -S 2048' -S
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Compiler: gcc 2.6.3
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Compiled: -O, without CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.20
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8192 INSERTs INTO SIMPLE (1 xact): 39.58
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8192 INSERTs INTO SIMPLE (8192 xacts): 1190.98
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Create INDEX on SIMPLE: 3.69
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 46.65
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8192 random INDEX scans on SIMPLE (1 xact): 65.49
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ORDER BY SIMPLE: 2.27
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2. run on leslie with asserts
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DBMS: PostgreSQL 6.3.2 (plus changes to 98/06/01)
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OS: Linux 2.0.32 leslie
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HardWare: i586/133 HX 512, 32M RAM, fast SCSI, 7200rpm
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StartUp: postmaster -B 256 '-o -S 2048' -S
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Compiler: gcc 2.7.2.3
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Compiled: -O, WITH CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.10
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8192 INSERTs INTO SIMPLE (1 xact): 15.48
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8192 INSERTs INTO SIMPLE (8192 xacts): 326.75
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Create INDEX on SIMPLE: 2.99
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 20.69
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8192 random INDEX scans on SIMPLE (1 xact): 35.81
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ORDER BY SIMPLE: 1.68
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3. with -F to avoid forced i/o
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DBMS: PostgreSQL 6.3.2 (plus changes to 98/06/01)
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OS: Linux 2.0.32 leslie
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HardWare: i586/133 HX 512, 32M RAM, fast SCSI, 7200rpm
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StartUp: postmaster -B 256 '-o -S 2048 -F' -S
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Compiler: gcc 2.7.2.3
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Compiled: -O, WITH CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.10
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8192 INSERTs INTO SIMPLE (1 xact): 15.90
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8192 INSERTs INTO SIMPLE (8192 xacts): 24.98
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Create INDEX on SIMPLE: 2.63
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 20.46
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8192 random INDEX scans on SIMPLE (1 xact): 36.43
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ORDER BY SIMPLE: 1.69
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4. no asserts, -F to avoid forced I/O
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DBMS: PostgreSQL 6.3.2 (plus changes to 98/06/01)
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OS: Linux 2.0.32 leslie
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HardWare: i586/133 HX 512, 32M RAM, fast SCSI, 7200rpm
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StartUp: postmaster -B 256 '-o -S 2048' -S
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Compiler: gcc 2.7.2.3
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Compiled: -O, No CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.10
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8192 INSERTs INTO SIMPLE (1 xact): 14.92
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8192 INSERTs INTO SIMPLE (8192 xacts): 23.23
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Create INDEX on SIMPLE: 1.38
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 18.67
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8192 random INDEX scans on SIMPLE (1 xact): 33.79
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ORDER BY SIMPLE: 1.58
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5. with more buffers (2048 vs 256) and -F to avoid forced i/o
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DBMS: PostgreSQL 6.3.2 (plus changes to 98/06/01)
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OS: Linux 2.0.32 leslie
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HardWare: i586/133 HX 512, 32M RAM, fast SCSI, 7200rpm
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StartUp: postmaster -B 2048 '-o -S 2048 -F' -S
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Compiler: gcc 2.7.2.3
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Compiled: -O, WITH CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.11
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8192 INSERTs INTO SIMPLE (1 xact): 21.31
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8192 INSERTs INTO SIMPLE (8192 xacts): 42.28
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Create INDEX on SIMPLE: 2.65
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 25.74
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8192 random INDEX scans on SIMPLE (1 xact): 42.26
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ORDER BY SIMPLE: 1.72
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6. No Asserts, more buffers (2048 vs 256) and -F to avoid forced i/o
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DBMS: PostgreSQL 6.3.2 (plus changes to 98/06/01)
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OS: Linux 2.0.32 leslie
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HardWare: i586/133 HX 512, 32M RAM, fast SCSI, 7200rpm
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StartUp: postmaster -B 2048 '-o -S 2048 -F' -S
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Compiler: gcc 2.7.2.3
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Compiled: -O, No CASSERT checking, with
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-DTBL_FREE_CMD_MEMORY (to free memory
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if BEGIN/END after each query execution)
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DB connection startup: 0.11
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8192 INSERTs INTO SIMPLE (1 xact): 20.52
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8192 INSERTs INTO SIMPLE (8192 xacts): 39.79
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Create INDEX on SIMPLE: 1.40
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8192 INSERTs INTO SIMPLE with INDEX (1 xact): 24.77
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8192 random INDEX scans on SIMPLE (1 xact): 39.51
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ORDER BY SIMPLE: 1.55
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---------------------------------------------------------------------
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-dg
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David Gould dg@illustra.com 510.628.3783 or 510.305.9468
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Informix Software (No, really) 300 Lakeside Drive Oakland, CA 94612
|
||
"Don't worry about people stealing your ideas. If your ideas are any
|
||
good, you'll have to ram them down people's throats." -- Howard Aiken
|
||
|
||
|
||
From owner-pgsql-hackers@hub.org Tue Oct 19 10:31:10 1999
|
||
Received: from renoir.op.net (root@renoir.op.net [209.152.193.4])
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Tue, 19 Oct 1999 10:09:15 -0400 (EDT)
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To: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
||
cc: "Vadim Mikheev" <vadim@krs.ru>, pgsql-hackers@postgreSQL.org
|
||
Subject: Re: [HACKERS] mdnblocks is an amazing time sink in huge relations
|
||
In-reply-to: Your message of Tue, 19 Oct 1999 19:03:22 +0900
|
||
<000801bf1a19$2d88ae20$2801007e@cadzone.tpf.co.jp>
|
||
Date: Tue, 19 Oct 1999 10:09:15 -0400
|
||
Message-ID: <9036.940342155@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
Sender: owner-pgsql-hackers@postgreSQL.org
|
||
Status: RO
|
||
|
||
"Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
|
||
> 1. shared cache holds committed system tuples.
|
||
> 2. private cache holds uncommitted system tuples.
|
||
> 3. relpages of shared cache are updated immediately by
|
||
> phisical change and corresponding buffer pages are
|
||
> marked dirty.
|
||
> 4. on commit, the contents of uncommitted tuples except
|
||
> relpages,reltuples,... are copied to correponding tuples
|
||
> in shared cache and the combined contents are
|
||
> committed.
|
||
> If so,catalog cache invalidation would be no longer needed.
|
||
> But synchronization of the step 4. may be difficult.
|
||
|
||
I think the main problem is that relpages and reltuples shouldn't
|
||
be kept in pg_class columns at all, because they need to have
|
||
very different update behavior from the other pg_class columns.
|
||
|
||
The rest of pg_class is update-on-commit, and we can lock down any one
|
||
row in the normal MVCC way (if transaction A has modified a row and
|
||
transaction B also wants to modify it, B waits for A to commit or abort,
|
||
so it can know which version of the row to start from). Furthermore,
|
||
there can legitimately be several different values of a row in use in
|
||
different places: the latest committed, an uncommitted modification, and
|
||
one or more old values that are still being used by active transactions
|
||
because they were current when those transactions started. (BTW, the
|
||
present relcache is pretty bad about maintaining pure MVCC transaction
|
||
semantics like this, but it seems clear to me that that's the direction
|
||
we want to go in.)
|
||
|
||
relpages cannot operate this way. To be useful for avoiding lseeks,
|
||
relpages *must* change exactly when the physical file changes. It
|
||
matters not at all whether the particular transaction that extended the
|
||
file ultimately commits or not. Moreover there can be only one correct
|
||
value (per relation) across the whole system, because there is only one
|
||
length of the relation file.
|
||
|
||
If we want to take reltuples seriously and try to maintain it
|
||
on-the-fly, then I think it needs still a third behavior. Clearly
|
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it cannot be updated using MVCC rules, or we lose all writer
|
||
concurrency (if A has added tuples to a rel, B would have to wait
|
||
for A to commit before it could update reltuples...). Furthermore
|
||
"updating" isn't a simple matter of storing what you think the new
|
||
value is; otherwise two transactions adding tuples in parallel would
|
||
leave the wrong answer after B commits and overwrites A's value.
|
||
I think it would work for each transaction to keep track of a net delta
|
||
in reltuples for each table it's changed (total tuples added less total
|
||
tuples deleted), and then atomically add that value to the table's
|
||
shared reltuples counter during commit. But that still leaves the
|
||
problem of how you use the counter during a transaction to get an
|
||
accurate answer to the question "If I scan this table now, how many tuples
|
||
will I see?" At the time the question is asked, the current shared
|
||
counter value might include the effects of transactions that have
|
||
committed since your transaction started, and therefore are not visible
|
||
under MVCC rules. I think getting the correct answer would involve
|
||
making an instantaneous copy of the current counter at the start of
|
||
your xact, and then adding your own private net-uncommitted-delta to
|
||
the saved shared counter value when asked the question. This doesn't
|
||
look real practical --- you'd have to save the reltuples counts of
|
||
*all* tables in the database at the start of each xact, on the off
|
||
chance that you might need them. Ugh. Perhaps someone has a better
|
||
idea. In any case, reltuples clearly needs different mechanisms than
|
||
the ordinary fields in pg_class do, because updating it will be a
|
||
performance bottleneck otherwise.
|
||
|
||
If we allow reltuples to be updated only by vacuum-like events, as
|
||
it is now, then I think keeping it in pg_class is still OK.
|
||
|
||
In short, it seems clear to me that relpages should be removed from
|
||
pg_class and kept somewhere else if we want to make it more reliable
|
||
than it is now, and the same for reltuples (but reltuples doesn't
|
||
behave the same as relpages, and probably ought to be handled
|
||
differently).
|
||
|
||
regards, tom lane
|
||
|
||
************
|
||
|
||
From owner-pgsql-hackers@hub.org Tue Oct 19 21:25:30 1999
|
||
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|
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||
id KAA01715; Wed, 20 Oct 1999 10:05:14 +0900
|
||
From: "Hiroshi Inoue" <Inoue@tpf.co.jp>
|
||
To: "Tom Lane" <tgl@sss.pgh.pa.us>
|
||
Cc: <pgsql-hackers@postgreSQL.org>
|
||
Subject: RE: [HACKERS] mdnblocks is an amazing time sink in huge relations
|
||
Date: Wed, 20 Oct 1999 10:09:13 +0900
|
||
Message-ID: <000501bf1a97$b925a860$2801007e@cadzone.tpf.co.jp>
|
||
MIME-Version: 1.0
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|
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|
||
Sender: owner-pgsql-hackers@postgreSQL.org
|
||
Status: RO
|
||
|
||
> -----Original Message-----
|
||
> From: Hiroshi Inoue [mailto:Inoue@tpf.co.jp]
|
||
> Sent: Tuesday, October 19, 1999 6:45 PM
|
||
> To: Tom Lane
|
||
> Cc: pgsql-hackers@postgreSQL.org
|
||
> Subject: RE: [HACKERS] mdnblocks is an amazing time sink in huge
|
||
> relations
|
||
>
|
||
>
|
||
> >
|
||
> > "Hiroshi Inoue" <Inoue@tpf.co.jp> writes:
|
||
>
|
||
> [snip]
|
||
>
|
||
> >
|
||
> > > Deletion is necessary only not to consume disk space.
|
||
> > >
|
||
> > > For example vacuum could remove not deleted files.
|
||
> >
|
||
> > Hmm ... interesting idea ... but I can hear the complaints
|
||
> > from users already...
|
||
> >
|
||
>
|
||
> My idea is only an analogy of PostgreSQL's simple recovery
|
||
> mechanism of tuples.
|
||
>
|
||
> And my main point is
|
||
> "delete fails after commit" doesn't harm the database
|
||
> except that not deleted files consume disk space.
|
||
>
|
||
> Of cource,it's preferable to delete relation files immediately
|
||
> after(or just when) commit.
|
||
> Useless files are visible though useless tuples are invisible.
|
||
>
|
||
|
||
Anyway I don't need "DROP TABLE inside transactions" now
|
||
and my idea is originally for that issue.
|
||
|
||
After a thought,I propose the following solution.
|
||
|
||
1. mdcreate() couldn't create existent relation files.
|
||
If the existent file is of length zero,we would overwrite
|
||
the file.(seems the comment in md.c says so but the
|
||
code doesn't do so).
|
||
If the file is an Index relation file,we would overwrite
|
||
the file.
|
||
|
||
2. mdunlink() couldn't unlink non-existent relation files.
|
||
mdunlink() doesn't call elog(ERROR) even if the file
|
||
doesn't exist,though I couldn't find where to change
|
||
now.
|
||
mdopen() doesn't call elog(ERROR) even if the file
|
||
doesn't exist and leaves the relation as CLOSED.
|
||
|
||
Comments ?
|
||
|
||
Regards.
|
||
|
||
Hiroshi Inoue
|
||
Inoue@tpf.co.jp
|
||
|
||
************
|
||
|
||
From pgsql-hackers-owner+M6267@hub.org Sun Aug 27 21:46:37 2000
|
||
Received: from hub.org (root@hub.org [216.126.84.1])
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||
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||
for <pgman@candle.pha.pa.us>; Sun, 27 Aug 2000 20:46:36 -0400 (EDT)
|
||
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for <pgsql-hackers@postgreSQL.org>; Sun, 27 Aug 2000 20:05:29 -0400 (EDT)
|
||
To: pgsql-hackers@postgreSQL.org
|
||
Subject: [HACKERS] Possible performance improvement: buffer replacement policy
|
||
Date: Sun, 27 Aug 2000 20:05:29 -0400
|
||
Message-ID: <1601.967421129@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Mailing-List: pgsql-hackers@postgresql.org
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@hub.org
|
||
Status: RO
|
||
|
||
Those of you with long memories may recall a benchmark that Edmund Mergl
|
||
drew our attention to back in May '99. That test showed extremely slow
|
||
performance for updating a table with many indexes (about 20). At the
|
||
time, it seemed the problem was due to bad performance of btree with
|
||
many equal keys, so I thought I'd go back and retry the benchmark after
|
||
this latest round of btree hackery.
|
||
|
||
The good news is that btree itself seems to be pretty well fixed; the
|
||
bad news is that the benchmark is still slow for large numbers of rows.
|
||
The problem is I/O: the CPU mostly sits idle waiting for the disk.
|
||
As best I can tell, the difficulty is that the working set of pages
|
||
needed to update this many indexes is too large compared to the number
|
||
of disk buffers Postgres is using. (I was running with -B 1000 and
|
||
looking at behavior for a 100000-row test table. This gave me a table
|
||
size of 3876 pages, plus 11526 pages in 20 indexes.)
|
||
|
||
Of course, there's only so much we can do when the number of buffers
|
||
is too small, but I still started to wonder if we are using the buffers
|
||
as effectively as we can. Some tracing showed that most of the pages
|
||
of the indexes were being read and written multiple times within a
|
||
single UPDATE query, while most of the pages of the table proper were
|
||
fetched and written only once. That says we're not using the buffers
|
||
as well as we could; the index pages are not being kept in memory when
|
||
they should be. In a query like this, we should displace main-table
|
||
pages sooner to allow keeping more index pages in cache --- but with
|
||
the simple LRU replacement method we use, once a page has been loaded
|
||
it will stay in cache for at least the next NBuffers (-B) page
|
||
references, no matter what. With a large NBuffers that's a long time.
|
||
|
||
I've come across an interesting article:
|
||
The LRU-K Page Replacement Algorithm For Database Disk Buffering
|
||
Elizabeth J. O'Neil, Patrick E. O'Neil, Gerhard Weikum
|
||
Proceedings of the 1993 ACM SIGMOD international conference
|
||
on Management of Data, May 1993
|
||
(If you subscribe to the ACM digital library, you can get a PDF of this
|
||
from there.) This article argues that standard LRU buffer management is
|
||
inherently not great for database caches, and that it's much better to
|
||
replace pages on the basis of time since the K'th most recent reference,
|
||
not just time since the most recent one. K=2 is enough to get most of
|
||
the benefit. The big win is that you are measuring an actual page
|
||
interreference time (between the last two references) and not just
|
||
dealing with a lower-bound guess on the interreference time. Frequently
|
||
used pages are thus much more likely to stay in cache.
|
||
|
||
It looks like it wouldn't take too much work to replace shared buffers
|
||
on the basis of LRU-2 instead of LRU, so I'm thinking about trying it.
|
||
|
||
Has anyone looked into this area? Is there a better method to try?
|
||
|
||
regards, tom lane
|
||
|
||
From prlw1@newn.cam.ac.uk Fri Jan 19 12:54:45 2001
|
||
Received: from henry.newn.cam.ac.uk (henry.newn.cam.ac.uk [131.111.204.130])
|
||
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|
||
for <pgman@candle.pha.pa.us>; Fri, 19 Jan 2001 12:54:44 -0500 (EST)
|
||
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|
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|
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|
||
id 14Jfj6-0001cq-00; Fri, 19 Jan 2001 17:53:28 +0000
|
||
Date: Fri, 19 Jan 2001 17:53:28 +0000
|
||
From: Patrick Welche <prlw1@newn.cam.ac.uk>
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgreSQL.org
|
||
Subject: Re: [HACKERS] Possible performance improvement: buffer replacement policy
|
||
Message-ID: <20010119175328.A6223@quartz.newn.cam.ac.uk>
|
||
Reply-To: prlw1@cam.ac.uk
|
||
References: <1601.967421129@sss.pgh.pa.us> <200101191703.MAA25873@candle.pha.pa.us>
|
||
Mime-Version: 1.0
|
||
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|
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|
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|
||
In-Reply-To: <200101191703.MAA25873@candle.pha.pa.us>; from pgman@candle.pha.pa.us on Fri, Jan 19, 2001 at 12:03:58PM -0500
|
||
Status: RO
|
||
|
||
On Fri, Jan 19, 2001 at 12:03:58PM -0500, Bruce Momjian wrote:
|
||
>
|
||
> Tom, did we ever test this? I think we did and found that it was the
|
||
> same or worse, right?
|
||
|
||
(Funnily enough, I just read that message:)
|
||
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
cc: pgsql-hackers@postgreSQL.org
|
||
Subject: Re: [HACKERS] Possible performance improvement: buffer replacement policy
|
||
In-reply-to: <200010161541.LAA06653@candle.pha.pa.us>
|
||
References: <200010161541.LAA06653@candle.pha.pa.us>
|
||
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
|
||
message dated "Mon, 16 Oct 2000 11:41:41 -0400"
|
||
Date: Mon, 16 Oct 2000 11:49:52 -0400
|
||
Message-ID: <26100.971711392@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Mailing-List: pgsql-hackers@postgresql.org
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@hub.org
|
||
Status: RO
|
||
Content-Length: 947
|
||
Lines: 19
|
||
|
||
Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
>> It looks like it wouldn't take too much work to replace shared buffers
|
||
>> on the basis of LRU-2 instead of LRU, so I'm thinking about trying it.
|
||
>>
|
||
>> Has anyone looked into this area? Is there a better method to try?
|
||
|
||
> Sounds like a perfect idea. Good luck. :-)
|
||
|
||
Actually, the idea went down in flames :-(, but I neglected to report
|
||
back to pghackers about it. I did do some code to manage buffers as
|
||
LRU-2. I didn't have any good performance test cases to try it with,
|
||
but Richard Brosnahan was kind enough to re-run the TPC tests previously
|
||
published by Great Bridge with that code in place. Wasn't any faster,
|
||
in fact possibly a little slower, likely due to the extra CPU time spent
|
||
on buffer freelist management. It's possible that other scenarios might
|
||
show a better result, but right now I feel pretty discouraged about the
|
||
LRU-2 idea and am not pursuing it.
|
||
|
||
regards, tom lane
|
||
|
||
|
||
From pgsql-hackers-owner+M3455@postgresql.org Fri Jan 19 13:18:12 2001
|
||
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|
||
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|
||
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|
||
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|
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|
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|
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|
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|
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|
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|
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|
||
From: "Mikheev, Vadim" <vmikheev@SECTORBASE.COM>
|
||
To: "'Tom Lane'" <tgl@sss.pgh.pa.us>, Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Cc: pgsql-hackers@postgresql.org
|
||
Subject: RE: [HACKERS] Possible performance improvement: buffer replacemen
|
||
t policy
|
||
Date: Fri, 19 Jan 2001 10:07:27 -0800
|
||
MIME-Version: 1.0
|
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|
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Status: RO
|
||
|
||
> > Tom, did we ever test this? I think we did and found that
|
||
> > it was the same or worse, right?
|
||
>
|
||
> I tried it and didn't see any noticeable improvement on the particular
|
||
> test case I was using, so I got discouraged and didn't pursue the idea
|
||
> further. I'd like to come back to it someday, though.
|
||
|
||
I don't know how much useful could be LRU-2 but with WAL we should try
|
||
to reuse undirty free buffers first, not dirty ones, just to postpone
|
||
writes as long as we can. (BTW, this is what Oracle does.)
|
||
So, we probably should put new unfree dirty buffer just before first
|
||
dirty one in LRU.
|
||
|
||
Vadim
|
||
|
||
From markw@mohawksoft.com Thu Jun 7 14:40:02 2001
|
||
Return-path: <markw@mohawksoft.com>
|
||
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|
||
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|
||
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|
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|
||
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|
||
Message-ID: <3B1FC9CB.57C72AD6@mohawksoft.com>
|
||
Date: Thu, 07 Jun 2001 14:36:59 -0400
|
||
From: mlw <markw@mohawksoft.com>
|
||
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|
||
X-Accept-Language: en
|
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MIME-Version: 1.0
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||
Subject: Re: 7.2 items
|
||
References: <200106071503.f57F32n03924@candle.pha.pa.us>
|
||
Content-Type: text/plain; charset=us-ascii
|
||
Content-Transfer-Encoding: 7bit
|
||
Status: RO
|
||
|
||
Bruce Momjian wrote:
|
||
|
||
> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> >
|
||
> > > Here is a small list of big TODO items. I was wondering which ones
|
||
> > > people were thinking about for 7.2?
|
||
> >
|
||
> > A friend of mine wants to use PostgreSQL instead of Oracle for a large
|
||
> > application, but has run into a snag when speed comparisons looked
|
||
> > good until the Oracle folks added a couple of BITMAP indexes. I can't
|
||
> > recall seeing any discussion about that here -- are there any plans?
|
||
>
|
||
> It is not on our list and I am not sure what they do.
|
||
|
||
Do you have access to any Oracle Documentation? There is a good explanation
|
||
of them.
|
||
|
||
However, I will try to explain.
|
||
|
||
If you have a table, locations. It has 1,000,000 records.
|
||
|
||
In oracle you do this:
|
||
|
||
create bitmap index bitmap_foo on locations (state) ;
|
||
|
||
For each unique value of 'state' oracle will create a bitmap with 1,000,000
|
||
bits in it. With a one representing a match and a zero representing no
|
||
match. Record '0' in the table is represented by bit '0' in the bitmap,
|
||
record '1' is represented by bit '1', record two by bit '2' and so on.
|
||
|
||
In a table where comparatively few different values are to be indexed in a
|
||
large table, a bitmap index can be quite small and not suffer the N * log(N)
|
||
disk I/O most tree based indexes suffer. If the bitmap is fairly sparse or
|
||
dense (or have periods of denseness and sparseness), it can be compressed
|
||
very efficiently as well.
|
||
|
||
When the statement:
|
||
|
||
select * from locations where state = 'MA';
|
||
|
||
Is executed, the bitmap is read into memory in very few disk operations.
|
||
(Perhaps even as few as one or two). It is a simple operation of rifling
|
||
through the bitmap for '1's that indicate the record has the property,
|
||
'state' = 'MA';
|
||
|
||
|
||
From mascarm@mascari.com Thu Jun 7 15:36:25 2001
|
||
Return-path: <mascarm@mascari.com>
|
||
Received: from corvette.mascari.com (dhcp065-024-161-045.columbus.rr.com [65.24.161.45])
|
||
by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f57JaOc21943
|
||
for <pgman@candle.pha.pa.us>; Thu, 7 Jun 2001 15:36:24 -0400 (EDT)
|
||
Received: from ferrari (ferrari.mascari.com [192.168.2.1])
|
||
by corvette.mascari.com (8.9.3/8.9.3) with SMTP id PAA25607;
|
||
Thu, 7 Jun 2001 15:29:31 -0400
|
||
Received: by localhost with Microsoft MAPI; Thu, 7 Jun 2001 15:34:18 -0400
|
||
Message-ID: <01C0EF67.5105D2E0.mascarm@mascari.com>
|
||
From: Mike Mascari <mascarm@mascari.com>
|
||
Reply-To: "mascarm@mascari.com" <mascarm@mascari.com>
|
||
To: "'mlw'" <markw@mohawksoft.com>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||
Subject: RE: [HACKERS] Re: 7.2 items
|
||
Date: Thu, 7 Jun 2001 15:34:17 -0400
|
||
Organization: Mascari Development Inc.
|
||
X-Mailer: Microsoft Internet E-mail/MAPI - 8.0.0.4211
|
||
MIME-Version: 1.0
|
||
Content-Type: text/plain; charset="us-ascii"
|
||
Content-Transfer-Encoding: 7bit
|
||
Status: RO
|
||
|
||
And in addition,
|
||
|
||
If you submitted the query:
|
||
|
||
SELECT * FROM addresses WHERE state = 'OH'
|
||
AND areacode = '614'
|
||
|
||
Then, with bitmap indexes, the bitmaps are just logically ANDed
|
||
together, and the final bitmap determines the matching rows.
|
||
|
||
Mike Mascari
|
||
mascarm@mascari.com
|
||
|
||
-----Original Message-----
|
||
From: mlw [SMTP:markw@mohawksoft.com]
|
||
|
||
Bruce Momjian wrote:
|
||
|
||
> > Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> >
|
||
> > > Here is a small list of big TODO items. I was wondering which
|
||
ones
|
||
> > > people were thinking about for 7.2?
|
||
> >
|
||
> > A friend of mine wants to use PostgreSQL instead of Oracle for a
|
||
large
|
||
> > application, but has run into a snag when speed comparisons
|
||
looked
|
||
> > good until the Oracle folks added a couple of BITMAP indexes. I
|
||
can't
|
||
> > recall seeing any discussion about that here -- are there any
|
||
plans?
|
||
>
|
||
> It is not on our list and I am not sure what they do.
|
||
|
||
Do you have access to any Oracle Documentation? There is a good
|
||
explanation
|
||
of them.
|
||
|
||
However, I will try to explain.
|
||
|
||
If you have a table, locations. It has 1,000,000 records.
|
||
|
||
In oracle you do this:
|
||
|
||
create bitmap index bitmap_foo on locations (state) ;
|
||
|
||
For each unique value of 'state' oracle will create a bitmap with
|
||
1,000,000
|
||
bits in it. With a one representing a match and a zero representing
|
||
no
|
||
match. Record '0' in the table is represented by bit '0' in the
|
||
bitmap,
|
||
record '1' is represented by bit '1', record two by bit '2' and so
|
||
on.
|
||
|
||
In a table where comparatively few different values are to be indexed
|
||
in a
|
||
large table, a bitmap index can be quite small and not suffer the N *
|
||
log(N)
|
||
disk I/O most tree based indexes suffer. If the bitmap is fairly
|
||
sparse or
|
||
dense (or have periods of denseness and sparseness), it can be
|
||
compressed
|
||
very efficiently as well.
|
||
|
||
When the statement:
|
||
|
||
select * from locations where state = 'MA';
|
||
|
||
Is executed, the bitmap is read into memory in very few disk
|
||
operations.
|
||
(Perhaps even as few as one or two). It is a simple operation of
|
||
rifling
|
||
through the bitmap for '1's that indicate the record has the
|
||
property,
|
||
'state' = 'MA';
|
||
|
||
|
||
|
||
From oleg@sai.msu.su Thu Jun 7 15:39:15 2001
|
||
Return-path: <oleg@sai.msu.su>
|
||
Received: from ra.sai.msu.su (ra.sai.msu.su [158.250.29.2])
|
||
by candle.pha.pa.us (8.10.1/8.10.1) with ESMTP id f57Jd7c22010
|
||
for <pgman@candle.pha.pa.us>; Thu, 7 Jun 2001 15:39:08 -0400 (EDT)
|
||
Received: from ra (ra [158.250.29.2])
|
||
by ra.sai.msu.su (8.9.3/8.9.3) with ESMTP id WAA07783;
|
||
Thu, 7 Jun 2001 22:38:20 +0300 (GMT)
|
||
Date: Thu, 7 Jun 2001 22:38:20 +0300 (GMT)
|
||
From: Oleg Bartunov <oleg@sai.msu.su>
|
||
X-X-Sender: <megera@ra.sai.msu.su>
|
||
To: mlw <markw@mohawksoft.com>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
"pgsql-hackers@postgresql.org" <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Re: 7.2 items
|
||
In-Reply-To: <3B1FC9CB.57C72AD6@mohawksoft.com>
|
||
Message-ID: <Pine.GSO.4.33.0106072234120.6015-100000@ra.sai.msu.su>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: RO
|
||
|
||
I think it's possible to implement bitmap indexes with a little
|
||
effort using GiST. at least I know one implementation
|
||
http://www.it.iitb.ernet.in/~rvijay/dbms/proj/
|
||
if you have interests you could implement bitmap indexes yourself
|
||
unfortunately, we're very busy
|
||
|
||
Oleg
|
||
On Thu, 7 Jun 2001, mlw wrote:
|
||
|
||
> Bruce Momjian wrote:
|
||
>
|
||
> > > Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> > >
|
||
> > > > Here is a small list of big TODO items. I was wondering which ones
|
||
> > > > people were thinking about for 7.2?
|
||
> > >
|
||
> > > A friend of mine wants to use PostgreSQL instead of Oracle for a large
|
||
> > > application, but has run into a snag when speed comparisons looked
|
||
> > > good until the Oracle folks added a couple of BITMAP indexes. I can't
|
||
> > > recall seeing any discussion about that here -- are there any plans?
|
||
> >
|
||
> > It is not on our list and I am not sure what they do.
|
||
>
|
||
> Do you have access to any Oracle Documentation? There is a good explanation
|
||
> of them.
|
||
>
|
||
> However, I will try to explain.
|
||
>
|
||
> If you have a table, locations. It has 1,000,000 records.
|
||
>
|
||
> In oracle you do this:
|
||
>
|
||
> create bitmap index bitmap_foo on locations (state) ;
|
||
>
|
||
> For each unique value of 'state' oracle will create a bitmap with 1,000,000
|
||
> bits in it. With a one representing a match and a zero representing no
|
||
> match. Record '0' in the table is represented by bit '0' in the bitmap,
|
||
> record '1' is represented by bit '1', record two by bit '2' and so on.
|
||
>
|
||
> In a table where comparatively few different values are to be indexed in a
|
||
> large table, a bitmap index can be quite small and not suffer the N * log(N)
|
||
> disk I/O most tree based indexes suffer. If the bitmap is fairly sparse or
|
||
> dense (or have periods of denseness and sparseness), it can be compressed
|
||
> very efficiently as well.
|
||
>
|
||
> When the statement:
|
||
>
|
||
> select * from locations where state = 'MA';
|
||
>
|
||
> Is executed, the bitmap is read into memory in very few disk operations.
|
||
> (Perhaps even as few as one or two). It is a simple operation of rifling
|
||
> through the bitmap for '1's that indicate the record has the property,
|
||
> 'state' = 'MA';
|
||
>
|
||
>
|
||
> ---------------------------(end of broadcast)---------------------------
|
||
> TIP 6: Have you searched our list archives?
|
||
>
|
||
> http://www.postgresql.org/search.mpl
|
||
>
|
||
|
||
Regards,
|
||
Oleg
|
||
_____________________________________________________________
|
||
Oleg Bartunov, sci.researcher, hostmaster of AstroNet,
|
||
Sternberg Astronomical Institute, Moscow University (Russia)
|
||
Internet: oleg@sai.msu.su, http://www.sai.msu.su/~megera/
|
||
phone: +007(095)939-16-83, +007(095)939-23-83
|
||
|
||
|
||
From pgsql-general-owner+M2497@hub.org Fri Jun 16 18:31:03 2000
|
||
Received: from renoir.op.net (root@renoir.op.net [207.29.195.4])
|
||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id RAA04165
|
||
for <pgman@candle.pha.pa.us>; Fri, 16 Jun 2000 17:31:01 -0400 (EDT)
|
||
Received: from hub.org (root@hub.org [216.126.84.1]) by renoir.op.net (o1/$Revision: 1.17 $) with ESMTP id RAA13110 for <pgman@candle.pha.pa.us>; Fri, 16 Jun 2000 17:20:12 -0400 (EDT)
|
||
Received: from hub.org (majordom@localhost [127.0.0.1])
|
||
by hub.org (8.10.1/8.10.1) with SMTP id e5GLDaM14477;
|
||
Fri, 16 Jun 2000 17:13:36 -0400 (EDT)
|
||
Received: from home.dialix.com ([203.15.150.26])
|
||
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|
||
for <pgsql-general@postgresql.org>; Fri, 16 Jun 2000 17:12:27 -0400 (EDT)
|
||
Received: from nemeton.com.au ([202.76.153.71])
|
||
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|
||
for <pgsql-general@postgresql.org>; Sat, 17 Jun 2000 07:11:44 +1000 (EST)
|
||
(envelope-from giles@nemeton.com.au)
|
||
Received: (qmail 10213 invoked from network); 16 Jun 2000 09:52:29 -0000
|
||
Received: from nemeton.com.au (203.8.3.17)
|
||
by nemeton.com.au with SMTP; 16 Jun 2000 09:52:29 -0000
|
||
To: Jurgen Defurne <defurnj@glo.be>
|
||
cc: Mark Stier <kalium@gmx.de>,
|
||
postgreSQL general mailing list <pgsql-general@postgresql.org>
|
||
Subject: Re: [GENERAL] optimization by removing the file system layer?
|
||
In-Reply-To: Message from Jurgen Defurne <defurnj@glo.be>
|
||
of "Thu, 15 Jun 2000 20:26:57 +0200." <39491FF1.E1E583F8@glo.be>
|
||
Date: Fri, 16 Jun 2000 19:52:28 +1000
|
||
Message-ID: <10210.961149148@nemeton.com.au>
|
||
From: Giles Lean <giles@nemeton.com.au>
|
||
X-Mailing-List: pgsql-general@postgresql.org
|
||
Precedence: bulk
|
||
Sender: pgsql-general-owner@hub.org
|
||
Status: OR
|
||
|
||
|
||
|
||
> I think that the Un*x filesystem is one of the reasons that large
|
||
> database vendors rather use raw devices, than filesystem storage
|
||
> files.
|
||
|
||
This used to be the preference, back in the late 80s and possibly
|
||
early 90s. I'm seeing a preference toward using the filesystem now,
|
||
possibly with some sort of async I/O and co-operation from the OS
|
||
filesystem about interactions with the filesystem cache.
|
||
|
||
Performance preferences don't stand still. The hardware changes, the
|
||
software changes, the volume of data changes, and different solutions
|
||
become preferable.
|
||
|
||
> Using a raw device on the disk gives them the possibility to have
|
||
> complete control over their files, indices and objects without being
|
||
> bothered by the operating system.
|
||
>
|
||
> This speeds up things in several ways :
|
||
> - the least possible OS intervention
|
||
|
||
Not that this is especially useful, necessarily. If the "raw" device
|
||
is in fact managed by a logical volume manager doing mirroring onto
|
||
some sort of storage array there is still plenty of OS code involved.
|
||
|
||
The cost of using a filesystem in addition may not be much if anything
|
||
and of course a filesystem is considerably more flexible to
|
||
administer (backup, move, change size, check integrity, etc.)
|
||
|
||
> - choose block sizes according to applications
|
||
> - reducing fragmentation
|
||
> - packing data in nearby cilinders
|
||
|
||
... but when this storage area is spread over multiple mechanisms in a
|
||
smart storage array with write caching, you've no idea what is where
|
||
anyway. Better to let the hardware or at least the OS manage this;
|
||
there are so many levels of caching between a database and the
|
||
magnetic media that working hard to influence layout is almost
|
||
certainly a waste of time.
|
||
|
||
Kirk McKusick tells a lovely story that once upon a time it used to be
|
||
sensible to check some registers on a particular disk controller to
|
||
find out where the heads were when scheduling I/O. Needless to say,
|
||
that is history now!
|
||
|
||
There's a considerable cost in complexity and code in using "raw"
|
||
storage too, and it's not a one off cost: as the technologies change,
|
||
the "fast" way to do things will change and the code will have to be
|
||
updated to match. Better to leave this to the OS vendor where
|
||
possible, and take advantage of the tuning they do.
|
||
|
||
> - Anyone other ideas -> the sky is the limit here
|
||
|
||
> It also aids portability, at least on platforms that have an
|
||
> equivalent of a raw device.
|
||
|
||
I don't understand that claim. Not much is portable about raw
|
||
devices, and they're typically not nearlly as well documented as the
|
||
filesystem interfaces.
|
||
|
||
> It is also independent of the standard implemented Un*x filesystems,
|
||
> for which you will have to pay extra if you want to take extra
|
||
> measures against power loss.
|
||
|
||
Rather, it is worse. With a Unix filesystem you get quite defined
|
||
semantics about what is written when.
|
||
|
||
> The problem with e.g. e2fs, is that it is not robust enough if a CPU
|
||
> fails.
|
||
|
||
ext2fs doesn't even claim to have Unix filesystem semantics.
|
||
|
||
Regards,
|
||
|
||
Giles
|
||
|
||
|
||
|
||
From pgsql-hackers-owner+M1795@postgresql.org Thu Dec 7 18:47:52 2000
|
||
Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28])
|
||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id SAA09172
|
||
for <pgman@candle.pha.pa.us>; Thu, 7 Dec 2000 18:47:52 -0500 (EST)
|
||
Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28])
|
||
by mail.postgresql.org (8.11.1/8.11.1) with SMTP id eB7NjFP10612;
|
||
Thu, 7 Dec 2000 18:45:15 -0500 (EST)
|
||
(envelope-from pgsql-hackers-owner+M1795@postgresql.org)
|
||
Received: from thor.tht.net (thor.tht.net [209.47.145.4])
|
||
by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id eB7N6BP08233
|
||
for <pgsql-hackers@postgresql.org>; Thu, 7 Dec 2000 18:06:11 -0500 (EST)
|
||
(envelope-from bright@fw.wintelcom.net)
|
||
Received: from fw.wintelcom.net (bright@ns1.wintelcom.net [209.1.153.20])
|
||
by thor.tht.net (8.9.3/8.9.3) with ESMTP id SAA97456
|
||
for <pgsql-hackers@postgresql.org>; Thu, 7 Dec 2000 18:57:32 GMT
|
||
(envelope-from bright@fw.wintelcom.net)
|
||
Received: (from bright@localhost)
|
||
by fw.wintelcom.net (8.10.0/8.10.0) id eB7MvWE21269
|
||
for pgsql-hackers@postgresql.org; Thu, 7 Dec 2000 14:57:32 -0800 (PST)
|
||
Date: Thu, 7 Dec 2000 14:57:32 -0800
|
||
From: Alfred Perlstein <bright@wintelcom.net>
|
||
To: pgsql-hackers@postgresql.org
|
||
Subject: [HACKERS] Patches with vacuum fixes available for 7.0.x
|
||
Message-ID: <20001207145732.X16205@fw.wintelcom.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: text/plain; charset=us-ascii
|
||
Content-Disposition: inline
|
||
User-Agent: Mutt/1.2.5i
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
Status: ORr
|
||
|
||
We recently had a very satisfactory contract completed by
|
||
Vadim.
|
||
|
||
Basically Vadim has been able to reduce the amount of time
|
||
taken by a vacuum from 10-15 minutes down to under 10 seconds.
|
||
|
||
We've been running with these patches under heavy load for
|
||
about a week now without any problems except one:
|
||
don't 'lazy' (new option for vacuum) a table which has just
|
||
had an index created on it, or at least don't expect it to
|
||
take any less time than a normal vacuum would.
|
||
|
||
There's three patchsets and they are available at:
|
||
|
||
http://people.freebsd.org/~alfred/vacfix/
|
||
|
||
complete diff:
|
||
http://people.freebsd.org/~alfred/vacfix/v.diff
|
||
|
||
only lazy vacuum option to speed up index vacuums:
|
||
http://people.freebsd.org/~alfred/vacfix/vlazy.tgz
|
||
|
||
only lazy vacuum option to only scan from start of modified
|
||
data:
|
||
http://people.freebsd.org/~alfred/vacfix/mnmb.tgz
|
||
|
||
Although the patches are for 7.0.x I'm hoping that they
|
||
can be forward ported (if Vadim hasn't done it already)
|
||
to 7.1.
|
||
|
||
enjoy!
|
||
|
||
--
|
||
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
|
||
"I have the heart of a child; I keep it in a jar on my desk."
|
||
|
||
From pgsql-hackers-owner+M1809@postgresql.org Thu Dec 7 20:27:39 2000
|
||
Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28])
|
||
by candle.pha.pa.us (8.9.0/8.9.0) with ESMTP id UAA11827
|
||
for <pgman@candle.pha.pa.us>; Thu, 7 Dec 2000 20:27:38 -0500 (EST)
|
||
Received: from mail.postgresql.org (webmail.postgresql.org [216.126.85.28])
|
||
by mail.postgresql.org (8.11.1/8.11.1) with SMTP id eB81PsP22362;
|
||
Thu, 7 Dec 2000 20:25:54 -0500 (EST)
|
||
(envelope-from pgsql-hackers-owner+M1809@postgresql.org)
|
||
Received: from fw.wintelcom.net (ns1.wintelcom.net [209.1.153.20])
|
||
by mail.postgresql.org (8.11.1/8.11.1) with ESMTP id eB81JkP21783
|
||
for <pgsql-hackers@postgresql.org>; Thu, 7 Dec 2000 20:19:46 -0500 (EST)
|
||
(envelope-from bright@fw.wintelcom.net)
|
||
Received: (from bright@localhost)
|
||
by fw.wintelcom.net (8.10.0/8.10.0) id eB81JwU25447;
|
||
Thu, 7 Dec 2000 17:19:58 -0800 (PST)
|
||
Date: Thu, 7 Dec 2000 17:19:58 -0800
|
||
From: Alfred Perlstein <bright@wintelcom.net>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Patches with vacuum fixes available for 7.0.x
|
||
Message-ID: <20001207171958.B16205@fw.wintelcom.net>
|
||
References: <20001207145732.X16205@fw.wintelcom.net> <28791.976236143@sss.pgh.pa.us>
|
||
MIME-Version: 1.0
|
||
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|
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|
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|
||
In-Reply-To: <28791.976236143@sss.pgh.pa.us>; from tgl@sss.pgh.pa.us on Thu, Dec 07, 2000 at 07:42:23PM -0500
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
Status: OR
|
||
|
||
* Tom Lane <tgl@sss.pgh.pa.us> [001207 17:10] wrote:
|
||
> Alfred Perlstein <bright@wintelcom.net> writes:
|
||
> > Basically Vadim has been able to reduce the amount of time
|
||
> > taken by a vacuum from 10-15 minutes down to under 10 seconds.
|
||
>
|
||
> Cool. What's it do, exactly?
|
||
|
||
================================================================
|
||
|
||
The first is a bonus that Vadim gave us to speed up index
|
||
vacuums, I'm not sure I understand it completely, but it
|
||
work really well. :)
|
||
|
||
here's the README he gave us:
|
||
|
||
Vacuum LAZY index cleanup option
|
||
|
||
LAZY vacuum option introduces new way of indices cleanup.
|
||
Instead of reading entire index file to remove index tuples
|
||
pointing to deleted table records, with LAZY option vacuum
|
||
performes index scans using keys fetched from table record
|
||
to be deleted. Vacuum checks each result returned by index
|
||
scan if it points to target heap record and removes
|
||
corresponding index tuple.
|
||
This can greatly speed up indices cleaning if not so many
|
||
table records were deleted/modified between vacuum runs.
|
||
Vacuum uses new option on user' demand.
|
||
|
||
New vacuum syntax is:
|
||
|
||
vacuum [verbose] [analyze] [lazy] [table [(columns)]]
|
||
|
||
================================================================
|
||
|
||
The second is one of the suggestions I gave on the lists a while
|
||
back, keeping track of the "last dirtied" block in the data files
|
||
to only scan the tail end of the file for deleted rows, I think
|
||
what he instead did was keep a table that holds all the modified
|
||
blocks and vacuum only scans those:
|
||
|
||
Minimal Number Modified Block (MNMB)
|
||
|
||
This feature is to track MNMB of required tables with triggers
|
||
to avoid reading unmodified table pages by vacuum. Triggers
|
||
store MNMB in per-table files in specified directory
|
||
($LIBDIR/contrib/mnmb by default) and create these files if not
|
||
existed.
|
||
|
||
Vacuum first looks up functions
|
||
|
||
mnmb_getblock(Oid databaseId, Oid tableId)
|
||
mnmb_setblock(Oid databaseId, Oid tableId, Oid block)
|
||
|
||
in catalog. If *both* functions were found *and* there was no
|
||
ANALYZE option specified then vacuum calls mnmb_getblock to obtain
|
||
MNMB for table being vacuumed and starts reading this table from
|
||
block number returned. After table was processed vacuum calls
|
||
mnmb_setblock to update data in file to last table block number.
|
||
Neither mnmb_getblock nor mnmb_setblock try to create file.
|
||
If there was no file for table being vacuumed then mnmb_getblock
|
||
returns 0 and mnmb_setblock does nothing.
|
||
mnmb_setblock() may be used to set in file MNMB to 0 and force
|
||
vacuum to read entire table if required.
|
||
|
||
To compile MNMB you have to add -DMNMB to CUSTOM_COPT
|
||
in src/Makefile.custom.
|
||
|
||
--
|
||
-Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
|
||
"I have the heart of a child; I keep it in a jar on my desk."
|
||
|
||
From pgsql-general-owner+M4010@postgresql.org Mon Feb 5 18:50:47 2001
|
||
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|
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(envelope-from news)
|
||
From: Mike Hoskins <mikehoskins@yahoo.com>
|
||
X-Newsgroups: comp.databases.postgresql.general
|
||
Subject: Re: [GENERAL] MySQL file system
|
||
Date: Tue, 30 Jan 2001 18:30:36 -0600
|
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|
||
Status: OR
|
||
|
||
This idea is such a popular (even old) one that Oracle developed it for 8i --
|
||
IFS. Yep, AS/400 has had it forever, and BeOS is another example. Informix has
|
||
had its DataBlades for years, as well. In fact, Reiser-FS is an FS implemented
|
||
on a DB, albeit probably not a SQL DB. AIX's LVM and JFS is extent/DB-based, as
|
||
well. Let's see now, why would all those guys do that? (Now, some of those that
|
||
aren't SQL-based probably won't allow SQL queries on files, so just think about
|
||
those that do, for a minute)....
|
||
|
||
Rather than asking why, a far better question is why not? There is SO much
|
||
functionality to be gained here that it's silly to ask why. At a higher level,
|
||
treating BLOBs as files and as DB entries simultaneously has so many uses, that
|
||
one has trouble answering the question properly without the puzzled stare back
|
||
at the questioner. Again, look at the above list, particularly at AS/400 -- the
|
||
entire OS's FS sits on top of DB/2!
|
||
|
||
For example, think how easy dynamically generated web sites could access online
|
||
catalog information, with all those JPEG's, GIFs, PNGs, HTML files, Text files,
|
||
.PDF's, etc., both in the DB and in the FS. This would be so much easier to
|
||
maintain, when you have webmasters, web designers, artists, programmers,
|
||
sysadmins, dba's, etc., all trying to manage a big, dynamic, graphics-rich web
|
||
site. Who cares if the FS is a bit slow, as long as it's not too slow? That's
|
||
not the point, anyway.
|
||
|
||
The point is easy access to data: asset management, version control, the
|
||
ability to access the same data as a file and as a BLOB simultaneously, the
|
||
ability to replicate easier, the ability to use more tools on the same info,
|
||
etc. It's not for speed, per se; instead, it's for accessibility.
|
||
|
||
Think about this issue. You have some already compiled text-based program that
|
||
works on binary files, but not on databases -- it was simply never designed into
|
||
the program. How are you going to get your graphics BLOBs into that program?
|
||
Oh yeah, let's write another program to transform our data into files, first,
|
||
then after processing delete them in some cleanup routine.... Why? If you have
|
||
a DB'ed FS, then file data can simultaneously have two views -- one for the DB
|
||
and one as an FS. (You can easily reverse the scenario.) Not only does this
|
||
save time and disk space; it saves you from having to pay for the most expensive
|
||
element of all -- programmer time.
|
||
|
||
BTW, once this FS-on-a-DB concept really sinks in, imagine how tightly
|
||
integrated Linux/Unix apps could be written. Imagine if a bunch of GPL'ed
|
||
software started coding for this and used this as a means to exchange data, all
|
||
using a common set of libraries. You could get to the point of uniting files,
|
||
BLOBs, data of all sorts, IPC, version control, etc., all under one umbrella,
|
||
especially if XML was the means data was exchanged. Heck, distributed
|
||
authentication, file access, data access, etc., could be improved greatly.
|
||
Well, this paragraph sounds like flame bait, but really consider the
|
||
ramifications. Also, read the next paragraph....
|
||
|
||
Something like this *has* existed for Postgres for a long time -- PGFS, by Brian
|
||
Bartholomew. It's even supposedly matured with age. Unfortunately, I cannot
|
||
get to http://www.wv.com/ (Working Version's main site). Working Version is a
|
||
version control system that keeps old versions of files around in the FS. It
|
||
uses PG as the back-end DB and lets you mount it like another FS. It's
|
||
supposedly an awesome system, but where is it? It's not some clunky korbit
|
||
thingy, either. (If someone can find it, please let me know by email, if
|
||
possible.)
|
||
|
||
The only thing I can find on this is from a Google search, which caches
|
||
everything but the actual software:
|
||
|
||
http://www.google.com/search?q=pgfs+postgres&num=100&hl=en&lr=lang_en&newwindow=1&safe=active
|
||
|
||
Also, there is the Perl-FS that can be transformed into something like PGFS:
|
||
http://www.assurdo.com/perlfs/ It allows you to write Perl code that can mount
|
||
various protocols or data types as an FS, in user space. (One example is the
|
||
ability to mount FTP sites, BTW.)
|
||
|
||
Instead of ridiculing something you've never tried, consider that MySQL-FS,
|
||
Oracle (IFS), Informix (DataBlades), AS/400 (DB/2), BeOS, and Reiser-FS are
|
||
doing this today. Do you want to be left behind and let them tell us what it's
|
||
good for? Or, do we want this for PG? (Reiser-FS, BTW, is FASTER than ext2,
|
||
but has no SQL hooks).
|
||
|
||
There were many posts on this on slashdot:
|
||
http://slashdot.org/article.pl?sid=01/01/16/1855253&mode=thread
|
||
(I wrote some comments here, as well, just look for mikehoskins)
|
||
|
||
I, for one, want to see this succeed for MySQL, PostgreSQL, msql, etc. It's an
|
||
awesome feature that doesn't need to be speedy because it can save HUMANS time.
|
||
|
||
The question really is, "When do we want to catch up to everyone else?" We are
|
||
always moving to higher levels of abstraction, anyway, so it's just a matter of
|
||
time. PG should participate.
|
||
|
||
|
||
Adam Lang wrote:
|
||
|
||
> I wasn't following the thread too closely, but database for a filesystem has
|
||
> been done. BeOS uses a database for a filesystem as well as AS/400 and
|
||
> Mainframes.
|
||
>
|
||
> Adam Lang
|
||
> Systems Engineer
|
||
> Rutgers Casualty Insurance Company
|
||
> http://www.rutgersinsurance.com
|
||
> ----- Original Message -----
|
||
> From: "Alfred Perlstein" <bright@wintelcom.net>
|
||
> To: "Robert D. Nelson" <RDNELSON@co.centre.pa.us>
|
||
> Cc: "Joseph Shraibman" <jks@selectacast.net>; "Karl DeBisschop"
|
||
> <karl@debisschop.net>; "Ned Lilly" <ned@greatbridge.com>; "PostgreSQL
|
||
> General" <pgsql-general@postgresql.org>
|
||
> Sent: Wednesday, January 17, 2001 12:23 PM
|
||
> Subject: Re: [GENERAL] MySQL file system
|
||
>
|
||
> > * Robert D. Nelson <RDNELSON@co.centre.pa.us> [010117 05:17] wrote:
|
||
> > > >Raw disk access allows:
|
||
> > >
|
||
> > > If I'm correct, mysql is providing a filesystem, not a way to access raw
|
||
> > > disk, like Oracle does. Huge difference there - with a filesystem, you
|
||
> have
|
||
> > > overhead of FS *and* SQL at the same time.
|
||
> >
|
||
> > Oh, so it's sort of like /proc for mysql?
|
||
> >
|
||
> > What a terrible waste of time and resources. :(
|
||
> >
|
||
> > --
|
||
> > -Alfred Perlstein - [bright@wintelcom.net|alfred@freebsd.org]
|
||
> > "I have the heart of a child; I keep it in a jar on my desk."
|
||
|
||
|
||
From pgsql-general-owner+M4049@postgresql.org Tue Feb 6 01:26:19 2001
|
||
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|
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Date: Tue, 06 Feb 2001 14:15:55 +0800
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To: Mike Hoskins <mikehoskins@yahoo.com>, pgsql-general@postgresql.org
|
||
From: Lincoln Yeoh <lyeoh@pop.jaring.my>
|
||
Subject: [GENERAL] Re: MySQL file system
|
||
In-Reply-To: <3A775CF7.3C5F1909@yahoo.com>
|
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|
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|
||
|
||
What you're saying seems to be to have a data structure where the same data
|
||
can be accessed in both the filesystem style and the RDBMs style. How does
|
||
that work? How is the mapping done between both structures? Slapping a
|
||
filesystem on top of a RDBMs doesn't do that does it?
|
||
|
||
Most filesystems are basically databases already, just differently
|
||
structured and featured databases. And so far most of them do their job
|
||
pretty well. You move a folder/directory somewhere, and everything inside
|
||
it moves. Tons of data are already arranged in that form. Though porting
|
||
over data from one filesystem to another is not always straightforward,
|
||
RDBMSes are far worse.
|
||
|
||
Maybe what would be nice is not a filesystem based on a database, rather
|
||
one influenced by databases. One with a decent fulltextindex for data and
|
||
filenames, where you have the option to ignore or not ignore
|
||
nonalphanumerics and still get an indexed search.
|
||
|
||
Then perhaps we could do something like the following:
|
||
|
||
select file.name from path "/var/logs/" where file.name like "%.log%' and
|
||
file.lastmodified > '2000/1/1' and file.contents =~ 'te_st[0-9]+\.gif$' use
|
||
index
|
||
|
||
Checkpoints would be nice too. Then I can rollback to a known point if I
|
||
screw up ;).
|
||
|
||
In fact the SQL style interface doesn't have to be built in at all. Neither
|
||
does the index have to be realtime. I suppose there could be an option to
|
||
make it realtime if performance is not an issue.
|
||
|
||
What could be done is to use some fast filesystem. Then we add tools to
|
||
maintain indexes, for SQL style interfaces and other style interfaces.
|
||
Checkpoints and rollbacks would be harder of course.
|
||
|
||
Cheerio,
|
||
Link.
|
||
|
||
|
||
From pgsql-hackers-owner+M20329@postgresql.org Tue Mar 19 18:00:15 2002
|
||
Return-path: <pgsql-hackers-owner+M20329@postgresql.org>
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|
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|
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|
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|
||
Subject: Re: [HACKERS] Bitmap indexes?
|
||
From: Greg Copeland <greg@CopelandConsulting.Net>
|
||
To: Matthew Kirkwood <matthew@hairy.beasts.org>
|
||
cc: Oleg Bartunov <oleg@sai.msu.su>,
|
||
PostgresSQL Hackers Mailing List <pgsql-hackers@postgresql.org>
|
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<Pine.LNX.4.33.0203192118140.29494-100000@sphinx.mythic-beasts.com>
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|
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|
||
On Tue, 2002-03-19 at 15:30, Matthew Kirkwood wrote:
|
||
> On Tue, 19 Mar 2002, Oleg Bartunov wrote:
|
||
>=20
|
||
> Sorry to reply over you, Oleg.
|
||
>=20
|
||
> > On 13 Mar 2002, Greg Copeland wrote:
|
||
> >
|
||
> > > One of the reasons why I originally stated following the hackers list=
|
||
is
|
||
> > > because I wanted to implement bitmap indexes. I found in the archive=
|
||
s,
|
||
> > > the follow link, http://www.it.iitb.ernet.in/~rvijay/dbms/proj/, which
|
||
> > > was extracted from this,
|
||
> > > http://groups.google.com/groups?hl=3Den&threadm=3D01C0EF67.5105D2E0.m=
|
||
ascarm%40mascari.com&rnum=3D1&prev=3D/groups%3Fq%3Dbitmap%2Bindex%2Bgroup:c=
|
||
omp.databases.postgresql.hackers%26hl%3Den%26selm%3D01C0EF67.5105D2E0.masca=
|
||
rm%2540mascari.com%26rnum%3D1, archive thread.
|
||
>=20
|
||
> For every case I have used a bitmap index on Oracle, a
|
||
> partial index[0] made more sense (especialy since it
|
||
> could usefully be compound).
|
||
|
||
That's very true, however, often bitmap indexes are used where partial
|
||
indexes may not work well. It maybe you were trying to apply the cure
|
||
for the wrong disease. ;)
|
||
|
||
>=20
|
||
> Our troublesome case (on Oracle) is a table of "events"
|
||
> where maybe fifty to a couple of hundred are "published"
|
||
> (ie. web-visible) at any time. The events are categorised
|
||
> by sport (about a dozen) and by "event type" (about five).
|
||
> We never really query events except by PK or by sport/type/
|
||
> published.
|
||
|
||
The reason why bitmap indexes are primarily used for DSS and data
|
||
wherehousing applications is because they are best used on extremely
|
||
large to very large tables which have low cardinality (e.g, 10,000,000
|
||
rows having 200 distinct values). On top of that, bitmap indexes also
|
||
tend to be much smaller than their "standard" cousins. On large and
|
||
very tables tables, this can sometimes save gigs in index space alone
|
||
(serious space benefit). Plus, their small index size tends to result
|
||
in much less I/O (serious speed benefit). This, of course, can result
|
||
in several orders of magnitude speed improvements when index scans are
|
||
required. As an added bonus, using AND, OR, XOR and NOT predicates are
|
||
exceptionally fast and if implemented properly, can even take advantage
|
||
of some 64-bit hardware for further speed improvements. This, of
|
||
course, further speeds look ups. The primary down side is that inserts
|
||
and updates to bitmap indexes are very costly (comparatively) which is,
|
||
yet again, why they excel in read-only environments (DSS & data
|
||
wherehousing).
|
||
|
||
It should also be noted that RDMS's, such as Oracle, often use multiple
|
||
types of bitmap indexes. This further impedes insert/update
|
||
performance, however, the additional bitmap index types usually allow
|
||
for range predicates while still making use of the bitmap index. If I'm
|
||
not mistaken, several other types of bitmaps are available as well as
|
||
many ways to encode and compress (rle, quad compression, etc) bitmap
|
||
indexes which further save on an already compact indexing scheme.
|
||
|
||
Given the proper problem domain, index bitmaps can be a big win.
|
||
|
||
>=20
|
||
> We make a bitmap index on "published", and trust Oracle to
|
||
> use it correctly, and hope that our other indexes are also
|
||
> useful.
|
||
>=20
|
||
> On Postgres[1] we would make a partial compound index:
|
||
>=20
|
||
> create index ... on events(sport_id,event_type_id)
|
||
> where published=3D'Y';
|
||
|
||
|
||
Generally speaking, bitmap indexes will not serve you very will on
|
||
tables having a low row counts, high cardinality or where they are
|
||
attached to tables which are primarily used in an OLTP capacity.=20
|
||
Situations where you have a low row count and low cardinality or high
|
||
row count and high cardinality tend to be better addressed by partial
|
||
indexes; which seem to make much more sense. In your example, it sounds
|
||
like you did "the right thing"(tm). ;)
|
||
|
||
|
||
Greg
|
||
|
||
|
||
--=-Ivchb84S75fOMzJ9DxwK
|
||
Content-Type: application/pgp-signature; name=signature.asc
|
||
Content-Description: This is a digitally signed message part
|
||
|
||
-----BEGIN PGP SIGNATURE-----
|
||
Version: GnuPG v1.0.6 (GNU/Linux)
|
||
Comment: For info see http://www.gnupg.org
|
||
|
||
iD8DBQA8l8Ml4lr1bpbcL6kRAhldAJ9Aoi9dwm1OteZjySfsd1o42trWLACfegQj
|
||
OEV6eO8MnBSlbJMHiQ08gNE=
|
||
=PQvW
|
||
-----END PGP SIGNATURE-----
|
||
|
||
--=-Ivchb84S75fOMzJ9DxwK--
|
||
|
||
|
||
From pgsql-hackers-owner+M26157@postgresql.org Tue Aug 6 23:06:34 2002
|
||
Date: Wed, 7 Aug 2002 13:07:38 +1000 (EST)
|
||
From: Gavin Sherry <swm@linuxworld.com.au>
|
||
To: Curt Sampson <cjs@cynic.net>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <Pine.NEB.4.44.0208071126590.1214-100000@angelic.cynic.net>
|
||
Message-ID: <Pine.LNX.4.21.0208071259210.13438-100000@linuxworld.com.au>
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Content-Length: 1357
|
||
|
||
On Wed, 7 Aug 2002, Curt Sampson wrote:
|
||
|
||
> But after doing some benchmarking of various sorts of random reads
|
||
> and writes, it occurred to me that there might be optimizations
|
||
> that could help a lot with this sort of thing. What if, when we've
|
||
> got an index block with a bunch of entries, instead of doing the
|
||
> reads in the order of the entries, we do them in the order of the
|
||
> blocks the entries point to? That would introduce a certain amount
|
||
> of "sequentialness" to the reads that the OS is not capable of
|
||
> introducing (since it can't reschedule the reads you're doing, the
|
||
> way it could reschedule, say, random writes).
|
||
|
||
This sounds more or less like the method employed by Firebird as described
|
||
by Ann Douglas to Tom at OSCON (correct me if I get this wrong).
|
||
|
||
Basically, firebird populates a bitmap with entries the scan is interested
|
||
in. The bitmap is populated in page order so that all entries on the same
|
||
heap page can be fetched at once.
|
||
|
||
This is totally different to the way postgres does things and would
|
||
require significant modification to the index access methods.
|
||
|
||
Gavin
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 3: if posting/reading through Usenet, please send an appropriate
|
||
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|
||
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|
||
|
||
From pgsql-hackers-owner+M26162@postgresql.org Wed Aug 7 00:42:35 2002
|
||
To: Curt Sampson <cjs@cynic.net>
|
||
cc: mark Kirkwood <markir@slithery.org>, Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <Pine.NEB.4.44.0208071126590.1214-100000@angelic.cynic.net>
|
||
References: <Pine.NEB.4.44.0208071126590.1214-100000@angelic.cynic.net>
|
||
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
|
||
message dated "Wed, 07 Aug 2002 11:31:32 +0900"
|
||
Date: Wed, 07 Aug 2002 00:41:47 -0400
|
||
Message-ID: <12593.1028695307@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Content-Length: 3063
|
||
|
||
Curt Sampson <cjs@cynic.net> writes:
|
||
> But after doing some benchmarking of various sorts of random reads
|
||
> and writes, it occurred to me that there might be optimizations
|
||
> that could help a lot with this sort of thing. What if, when we've
|
||
> got an index block with a bunch of entries, instead of doing the
|
||
> reads in the order of the entries, we do them in the order of the
|
||
> blocks the entries point to?
|
||
|
||
I thought to myself "didn't I just post something about that?"
|
||
and then realized it was on a different mailing list. Here ya go
|
||
(and no, this is not the first time around on this list either...)
|
||
|
||
|
||
I am currently thinking that bitmap indexes per se are not all that
|
||
interesting. What does interest me is bitmapped index lookup, which
|
||
came back into mind after hearing Ann Harrison describe how FireBird/
|
||
InterBase does it.
|
||
|
||
The idea is that you don't scan the index and base table concurrently
|
||
as we presently do it. Instead, you scan the index and make a list
|
||
of the TIDs of the table tuples you need to visit. This list can
|
||
be conveniently represented as a sparse bitmap. After you've finished
|
||
looking at the index, you visit all the required table tuples *in
|
||
physical order* using the bitmap. This eliminates multiple fetches
|
||
of the same heap page, and can possibly let you get some win from
|
||
sequential access.
|
||
|
||
Once you have built this mechanism, you can then move on to using
|
||
multiple indexes in interesting ways: you can do several indexscans
|
||
in one query and then AND or OR their bitmaps before doing the heap
|
||
scan. This would allow, for example, "WHERE a = foo and b = bar"
|
||
to be handled by ANDing results from separate indexes on the a and b
|
||
columns, rather than having to choose only one index to use as we do
|
||
now.
|
||
|
||
Some thoughts about implementation: FireBird's implementation seems
|
||
to depend on an assumption about a fixed number of tuple pointers
|
||
per page. We don't have that, but we could probably get away with
|
||
just allocating BLCKSZ/sizeof(HeapTupleHeaderData) bits per page.
|
||
Also, the main downside of this approach is that the bitmap could
|
||
get large --- but you could have some logic that causes you to fall
|
||
back to plain sequential scan if you get too many index hits. (It's
|
||
interesting to think of this as lossy compression of the bitmap...
|
||
which leads to the idea of only being fuzzy in limited areas of the
|
||
bitmap, rather than losing all the information you have.)
|
||
|
||
A possibly nasty issue is that lazy VACUUM has some assumptions in it
|
||
about indexscans holding pins on index pages --- that's what prevents
|
||
it from removing heap tuples that a concurrent indexscan is just about
|
||
to visit. It might be that there is no problem: even if lazy VACUUM
|
||
removes a heap tuple and someone else then installs a new tuple in that
|
||
same TID slot, you should be okay because the new tuple is too new to
|
||
pass your visibility test. But I'm not convinced this is safe.
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 6: Have you searched our list archives?
|
||
|
||
http://archives.postgresql.org
|
||
|
||
From pgsql-hackers-owner+M26172@postgresql.org Wed Aug 7 02:49:56 2002
|
||
X-Authentication-Warning: rh72.home.ee: hannu set sender to hannu@tm.ee using -f
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
From: Hannu Krosing <hannu@tm.ee>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Curt Sampson <cjs@cynic.net>, mark Kirkwood <markir@slithery.org>,
|
||
Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
In-Reply-To: <12776.1028697148@sss.pgh.pa.us>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
<12776.1028697148@sss.pgh.pa.us>
|
||
X-Mailer: Ximian Evolution 1.0.7
|
||
Date: 07 Aug 2002 09:46:29 +0500
|
||
Message-ID: <1028695589.2133.11.camel@rh72.home.ee>
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Content-Length: 1064
|
||
|
||
On Wed, 2002-08-07 at 10:12, Tom Lane wrote:
|
||
> Curt Sampson <cjs@cynic.net> writes:
|
||
> > On Wed, 7 Aug 2002, Tom Lane wrote:
|
||
> >> Also, the main downside of this approach is that the bitmap could
|
||
> >> get large --- but you could have some logic that causes you to fall
|
||
> >> back to plain sequential scan if you get too many index hits.
|
||
>
|
||
> > Well, what I was thinking of, should the list of TIDs to fetch get too
|
||
> > long, was just to break it down in to chunks.
|
||
>
|
||
> But then you lose the possibility of combining multiple indexes through
|
||
> bitmap AND/OR steps, which seems quite interesting to me. If you've
|
||
> visited only a part of each index then you can't apply that concept.
|
||
|
||
When the tuples are small relative to pagesize, you may get some
|
||
"compression" by saving just pages and not the actual tids in the the
|
||
bitmap.
|
||
|
||
-------------
|
||
Hannu
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 2: you can get off all lists at once with the unregister command
|
||
(send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
|
||
|
||
From pgsql-hackers-owner+M26166@postgresql.org Wed Aug 7 00:55:52 2002
|
||
Date: Wed, 7 Aug 2002 13:55:41 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: mark Kirkwood <markir@slithery.org>, Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <12593.1028695307@sss.pgh.pa.us>
|
||
Message-ID: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Content-Length: 1840
|
||
|
||
On Wed, 7 Aug 2002, Tom Lane wrote:
|
||
|
||
> I thought to myself "didn't I just post something about that?"
|
||
> and then realized it was on a different mailing list. Here ya go
|
||
> (and no, this is not the first time around on this list either...)
|
||
|
||
Wow. I'm glad to see you looking at this, because this feature would so
|
||
*so* much for the performance of some of my queries, and really, really
|
||
impress my "billion-row-database" client.
|
||
|
||
> The idea is that you don't scan the index and base table concurrently
|
||
> as we presently do it. Instead, you scan the index and make a list
|
||
> of the TIDs of the table tuples you need to visit.
|
||
|
||
Right.
|
||
|
||
> Also, the main downside of this approach is that the bitmap could
|
||
> get large --- but you could have some logic that causes you to fall
|
||
> back to plain sequential scan if you get too many index hits.
|
||
|
||
Well, what I was thinking of, should the list of TIDs to fetch get too
|
||
long, was just to break it down in to chunks. If you want to limit to,
|
||
say, 1000 TIDs, and your index has 3000, just do the first 1000, then
|
||
the next 1000, then the last 1000. This would still result in much less
|
||
disk head movement and speed the query immensely.
|
||
|
||
(BTW, I have verified this emperically during testing of random read vs.
|
||
random write on a RAID controller. The writes were 5-10 times faster
|
||
than the reads because the controller was caching a number of writes and
|
||
then doing them in the best possible order, whereas the reads had to be
|
||
satisfied in the order they were submitted to the controller.)
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 5: Have you checked our extensive FAQ?
|
||
|
||
http://www.postgresql.org/users-lounge/docs/faq.html
|
||
|
||
From pgsql-hackers-owner+M26167@postgresql.org Wed Aug 7 01:12:54 2002
|
||
To: Curt Sampson <cjs@cynic.net>
|
||
cc: mark Kirkwood <markir@slithery.org>, Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
|
||
message dated "Wed, 07 Aug 2002 13:55:41 +0900"
|
||
Date: Wed, 07 Aug 2002 01:12:28 -0400
|
||
Message-ID: <12776.1028697148@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
X-Virus-Scanned: by AMaViS new-20020517
|
||
Content-Length: 1428
|
||
|
||
Curt Sampson <cjs@cynic.net> writes:
|
||
> On Wed, 7 Aug 2002, Tom Lane wrote:
|
||
>> Also, the main downside of this approach is that the bitmap could
|
||
>> get large --- but you could have some logic that causes you to fall
|
||
>> back to plain sequential scan if you get too many index hits.
|
||
|
||
> Well, what I was thinking of, should the list of TIDs to fetch get too
|
||
> long, was just to break it down in to chunks.
|
||
|
||
But then you lose the possibility of combining multiple indexes through
|
||
bitmap AND/OR steps, which seems quite interesting to me. If you've
|
||
visited only a part of each index then you can't apply that concept.
|
||
|
||
Another point to keep in mind is that the bigger the bitmap gets, the
|
||
less useful an indexscan is, by definition --- sooner or later you might
|
||
as well fall back to a seqscan. So the idea of lossy compression of a
|
||
large bitmap seems really ideal to me. In principle you could seqscan
|
||
the parts of the table where matching tuples are thick on the ground,
|
||
and indexscan the parts where they ain't. Maybe this seems natural
|
||
to me as an old JPEG campaigner, but if you don't see the logic I
|
||
recommend thinking about it a little ...
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 3: if posting/reading through Usenet, please send an appropriate
|
||
subscribe-nomail command to majordomo@postgresql.org so that your
|
||
message can get through to the mailing list cleanly
|
||
|
||
From tgl@sss.pgh.pa.us Wed Aug 7 09:27:05 2002
|
||
To: Hannu Krosing <hannu@tm.ee>
|
||
cc: Curt Sampson <cjs@cynic.net>, mark Kirkwood <markir@slithery.org>,
|
||
Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <1028726966.13418.12.camel@taru.tm.ee>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net> <12776.1028697148@sss.pgh.pa.us> <1028695589.2133.11.camel@rh72.home.ee> <1028726966.13418.12.camel@taru.tm.ee>
|
||
Comments: In-reply-to Hannu Krosing <hannu@tm.ee>
|
||
message dated "07 Aug 2002 15:29:26 +0200"
|
||
Date: Wed, 07 Aug 2002 09:26:42 -0400
|
||
Message-ID: <15010.1028726802@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
Content-Length: 1120
|
||
|
||
Hannu Krosing <hannu@tm.ee> writes:
|
||
> Now I remembered my original preference for page bitmaps (vs. tuple
|
||
> bitmaps): one can't actually make good use of a bitmap of tuples because
|
||
> there is no fixed tuples/page ratio and thus no way to quickly go from
|
||
> bit position to actual tuple. You mention the same problem but propose a
|
||
> different solution.
|
||
|
||
> Using page bitmap, we will at least avoid fetching any unneeded pages -
|
||
> essentially we will have a sequential scan over possibly interesting
|
||
> pages.
|
||
|
||
Right. One form of the "lossy compression" idea I suggested is to
|
||
switch from a per-tuple bitmap to a per-page bitmap once the bitmap gets
|
||
too large to work with. Again, one could imagine doing that only in
|
||
denser areas of the bitmap.
|
||
|
||
> But I guess that CLUSTER support for INSERT will not be touched for 7.3
|
||
> as will real bitmap indexes ;)
|
||
|
||
All of this is far-future work I think. Adding a new scan type to the
|
||
executor would probably be pretty localized, but the ramifications in
|
||
the planner could be extensive --- especially if you want to do plans
|
||
involving ANDed or ORed bitmaps.
|
||
|
||
regards, tom lane
|
||
|
||
From pgsql-hackers-owner+M26178@postgresql.org Wed Aug 7 08:28:14 2002
|
||
X-Authentication-Warning: taru.tm.ee: hannu set sender to hannu@tm.ee using -f
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
From: Hannu Krosing <hannu@tm.ee>
|
||
To: Hannu Krosing <hannu@tm.ee>
|
||
cc: Tom Lane <tgl@sss.pgh.pa.us>, Curt Sampson <cjs@cynic.net>,
|
||
mark Kirkwood <markir@slithery.org>, Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
In-Reply-To: <1028695589.2133.11.camel@rh72.home.ee>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
<12776.1028697148@sss.pgh.pa.us> <1028695589.2133.11.camel@rh72.home.ee>
|
||
X-Mailer: Ximian Evolution 1.0.3.99
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Date: 07 Aug 2002 15:29:26 +0200
|
||
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|
||
X-Virus-Scanned: by AMaViS new-20020517
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|
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|
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|
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|
||
|
||
On Wed, 2002-08-07 at 06:46, Hannu Krosing wrote:
|
||
> On Wed, 2002-08-07 at 10:12, Tom Lane wrote:
|
||
> > Curt Sampson <cjs@cynic.net> writes:
|
||
> > > On Wed, 7 Aug 2002, Tom Lane wrote:
|
||
> > >> Also, the main downside of this approach is that the bitmap could
|
||
> > >> get large --- but you could have some logic that causes you to fall
|
||
> > >> back to plain sequential scan if you get too many index hits.
|
||
> >
|
||
> > > Well, what I was thinking of, should the list of TIDs to fetch get too
|
||
> > > long, was just to break it down in to chunks.
|
||
> >
|
||
> > But then you lose the possibility of combining multiple indexes through
|
||
> > bitmap AND/OR steps, which seems quite interesting to me. If you've
|
||
> > visited only a part of each index then you can't apply that concept.
|
||
>
|
||
> When the tuples are small relative to pagesize, you may get some
|
||
> "compression" by saving just pages and not the actual tids in the the
|
||
> bitmap.
|
||
|
||
Now I remembered my original preference for page bitmaps (vs. tuple
|
||
bitmaps): one can't actually make good use of a bitmap of tuples because
|
||
there is no fixed tuples/page ratio and thus no way to quickly go from
|
||
bit position to actual tuple. You mention the same problem but propose a
|
||
different solution.
|
||
|
||
Using page bitmap, we will at least avoid fetching any unneeded pages -
|
||
essentially we will have a sequential scan over possibly interesting
|
||
pages.
|
||
|
||
If we were to use page-bitmap index for something with only a few values
|
||
like booleans, some insert-time local clustering should be useful, so
|
||
that TRUEs and FALSEs end up on different pages.
|
||
|
||
But I guess that CLUSTER support for INSERT will not be touched for 7.3
|
||
as will real bitmap indexes ;)
|
||
|
||
---------------
|
||
Hannu
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 6: Have you searched our list archives?
|
||
|
||
http://archives.postgresql.org
|
||
|
||
From pgsql-hackers-owner+M26192@postgresql.org Wed Aug 7 10:26:30 2002
|
||
To: Hannu Krosing <hannu@tm.ee>
|
||
cc: Curt Sampson <cjs@cynic.net>, mark Kirkwood <markir@slithery.org>,
|
||
Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
In-Reply-To: <1028733234.13418.113.camel@taru.tm.ee>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net> <12776.1028697148@sss.pgh.pa.us> <1028695589.2133.11.camel@rh72.home.ee> <1028726966.13418.12.camel@taru.tm.ee> <15010.1028726802@sss.pgh.pa.us> <1028733234.13418.113.camel@taru.tm.ee>
|
||
Comments: In-reply-to Hannu Krosing <hannu@tm.ee>
|
||
message dated "07 Aug 2002 17:13:54 +0200"
|
||
Date: Wed, 07 Aug 2002 10:26:13 -0400
|
||
Message-ID: <15622.1028730373@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by AMaViS new-20020517
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Precedence: bulk
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|
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|
||
|
||
Hannu Krosing <hannu@tm.ee> writes:
|
||
> On Wed, 2002-08-07 at 15:26, Tom Lane wrote:
|
||
>> Right. One form of the "lossy compression" idea I suggested is to
|
||
>> switch from a per-tuple bitmap to a per-page bitmap once the bitmap gets
|
||
>> too large to work with.
|
||
|
||
> If it is a real bitmap, should it not be easyeast to allocate at the
|
||
> start ?
|
||
|
||
But it isn't a "real bitmap". That would be a really poor
|
||
implementation, both for space and speed --- do you really want to scan
|
||
over a couple of megs of zeroes to find the few one-bits you care about,
|
||
in the typical case? "Bitmap" is a convenient term because it describes
|
||
the abstract behavior we want, but the actual data structure will
|
||
probably be nontrivial. If I recall Ann's description correctly,
|
||
Firebird's implementation uses run length coding of some kind (anyone
|
||
care to dig in their source and get all the details?). If we tried
|
||
anything in the way of lossy compression then there'd be even more stuff
|
||
lurking under the hood.
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 2: you can get off all lists at once with the unregister command
|
||
(send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
|
||
|
||
From pgsql-hackers-owner+M26188@postgresql.org Wed Aug 7 10:12:26 2002
|
||
X-Authentication-Warning: taru.tm.ee: hannu set sender to hannu@tm.ee using -f
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
From: Hannu Krosing <hannu@tm.ee>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Curt Sampson <cjs@cynic.net>, mark Kirkwood <markir@slithery.org>,
|
||
Gavin Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
In-Reply-To: <15010.1028726802@sss.pgh.pa.us>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
<12776.1028697148@sss.pgh.pa.us> <1028695589.2133.11.camel@rh72.home.ee>
|
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<1028726966.13418.12.camel@taru.tm.ee> <15010.1028726802@sss.pgh.pa.us>
|
||
X-Mailer: Ximian Evolution 1.0.3.99
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|
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|
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X-Virus-Scanned: by AMaViS new-20020517
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|
||
|
||
On Wed, 2002-08-07 at 15:26, Tom Lane wrote:
|
||
> Hannu Krosing <hannu@tm.ee> writes:
|
||
> > Now I remembered my original preference for page bitmaps (vs. tuple
|
||
> > bitmaps): one can't actually make good use of a bitmap of tuples because
|
||
> > there is no fixed tuples/page ratio and thus no way to quickly go from
|
||
> > bit position to actual tuple. You mention the same problem but propose a
|
||
> > different solution.
|
||
>
|
||
> > Using page bitmap, we will at least avoid fetching any unneeded pages -
|
||
> > essentially we will have a sequential scan over possibly interesting
|
||
> > pages.
|
||
>
|
||
> Right. One form of the "lossy compression" idea I suggested is to
|
||
> switch from a per-tuple bitmap to a per-page bitmap once the bitmap gets
|
||
> too large to work with.
|
||
|
||
If it is a real bitmap, should it not be easyeast to allocate at the
|
||
start ?
|
||
|
||
a page bitmap for a 100 000 000 tuple table with 10 tuples/page will be
|
||
sized 10000000/8 = 1.25 MB, which does not look too big for me for that
|
||
amount of data (the data table itself would occupy 80 GB).
|
||
|
||
Even having the bitmap of 16 bits/page (with the bits 0-14 meaning
|
||
tuples 0-14 and bit 15 meaning "seq scan the rest of page") would
|
||
consume just 20 MB of _local_ memory, and would be quite justifyiable
|
||
for a query on a table that large.
|
||
|
||
For a real bitmap index the tuples-per-page should be a user-supplied
|
||
tuning parameter.
|
||
|
||
> Again, one could imagine doing that only in denser areas of the bitmap.
|
||
|
||
I would hardly call the resulting structure "a bitmap" ;)
|
||
|
||
And I'm not sure the overhead for a more complex structure would win us
|
||
any additional performance for most cases.
|
||
|
||
> > But I guess that CLUSTER support for INSERT will not be touched for 7.3
|
||
> > as will real bitmap indexes ;)
|
||
>
|
||
> All of this is far-future work I think.
|
||
|
||
After we do that we will probably be able claim support for
|
||
"datawarehousing" ;)
|
||
|
||
> Adding a new scan type to the
|
||
> executor would probably be pretty localized, but the ramifications in
|
||
> the planner could be extensive --- especially if you want to do plans
|
||
> involving ANDed or ORed bitmaps.
|
||
|
||
Also going to "smart inserter" which can do local clustering on sets of
|
||
real bitmap indexes for INSERTS (and INSERT side of UPDATE) would
|
||
probably be a major change from our current "stupid inserter" ;)
|
||
|
||
This will not be needed for bitmap resolution higher than 1bit/page but
|
||
default local clustering on bitmap indexes will probably buy us some
|
||
extra performance. by avoiding data page fetches when such indexes are
|
||
used.
|
||
|
||
AN anyway the support for INSERT being aware of clustering will probably
|
||
come up sometime.
|
||
|
||
------------
|
||
Hannu
|
||
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 2: you can get off all lists at once with the unregister command
|
||
(send "unregister YourEmailAddressHere" to majordomo@postgresql.org)
|
||
|
||
From hannu@tm.ee Wed Aug 7 11:22:53 2002
|
||
X-Authentication-Warning: taru.tm.ee: hannu set sender to hannu@tm.ee using -f
|
||
Subject: Re: [HACKERS] CLUSTER and indisclustered
|
||
From: Hannu Krosing <hannu@tm.ee>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Curt Sampson <cjs@cynic.net>, mark Kirkwood <markir@slithery.org>,
|
||
Gavin
|
||
Sherry <swm@linuxworld.com.au>,
|
||
Bruce Momjian <pgman@candle.pha.pa.us>, pgsql-hackers@postgresql.org
|
||
In-Reply-To: <15622.1028730373@sss.pgh.pa.us>
|
||
References: <Pine.NEB.4.44.0208071351440.1214-100000@angelic.cynic.net>
|
||
<12776.1028697148@sss.pgh.pa.us> <1028695589.2133.11.camel@rh72.home.ee>
|
||
<1028726966.13418.12.camel@taru.tm.ee> <15010.1028726802@sss.pgh.pa.us>
|
||
<1028733234.13418.113.camel@taru.tm.ee> <15622.1028730373@sss.pgh.pa.us>
|
||
X-Mailer: Ximian Evolution 1.0.3.99
|
||
Date: 07 Aug 2002 18:24:30 +0200
|
||
Message-ID: <1028737470.13419.182.camel@taru.tm.ee>
|
||
Content-Length: 2382
|
||
|
||
On Wed, 2002-08-07 at 16:26, Tom Lane wrote:
|
||
> Hannu Krosing <hannu@tm.ee> writes:
|
||
> > On Wed, 2002-08-07 at 15:26, Tom Lane wrote:
|
||
> >> Right. One form of the "lossy compression" idea I suggested is to
|
||
> >> switch from a per-tuple bitmap to a per-page bitmap once the bitmap gets
|
||
> >> too large to work with.
|
||
>
|
||
> > If it is a real bitmap, should it not be easyeast to allocate at the
|
||
> > start ?
|
||
>
|
||
> But it isn't a "real bitmap". That would be a really poor
|
||
> implementation, both for space and speed --- do you really want to scan
|
||
> over a couple of megs of zeroes to find the few one-bits you care about,
|
||
> in the typical case?
|
||
|
||
I guess that depends on data. The typical case should be somthing the
|
||
stats process will find out so the optimiser can use it
|
||
|
||
The bitmap must be less than 1/48 (size of TID) full for best
|
||
uncompressed "active-tid-list" to be smaller than plain bitmap. If there
|
||
were some structure above list then this ratio would be even higher.
|
||
|
||
I have had good experience using "compressed delta lists", which will
|
||
scale well ofer the whole "fullness" spectrum of bitmap, but this is for
|
||
storage, not for initial constructing of lists.
|
||
|
||
> "Bitmap" is a convenient term because it describes
|
||
> the abstract behavior we want, but the actual data structure will
|
||
> probably be nontrivial. If I recall Ann's description correctly,
|
||
> Firebird's implementation uses run length coding of some kind (anyone
|
||
> care to dig in their source and get all the details?).
|
||
|
||
Plain RLL is probably a good way to store it and for merging two or more
|
||
bitmaps, but not as good for constructing it bit-by-bit. I guess the
|
||
most effective structure for updating is often still a plain bitmap
|
||
(maybe not if it is very sparse and all of it does not fit in cache),
|
||
followed by some kind of balanced tree (maybe rb-tree).
|
||
|
||
If the bitmap is relatively full then the plain bitmap is almost always
|
||
the most effective to update.
|
||
|
||
> If we tried anything in the way of lossy compression then there'd
|
||
> be even more stuff lurking under the hood.
|
||
|
||
Having three-valued (0,1,maybe) RLL-encoded "tritmap" would be a good
|
||
way to represent lossy compression, and it would also be quite
|
||
straightforward to merge two of these using AND or OR. It may even be
|
||
possible to easily construct it using a fixed-length b-tree and going
|
||
from 1 to "maybe" for nodes that get too dense.
|
||
|
||
---------------
|
||
Hannu
|
||
|
||
|
||
From pgsql-hackers-owner+M21991@postgresql.org Wed Apr 24 23:37:37 2002
|
||
Return-path: <pgsql-hackers-owner+M21991@postgresql.org>
|
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Received: from postgresql.org (postgresql.org [64.49.215.8])
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To: tgl@sss.pgh.pa.us
|
||
cc: cjs@cynic.net, pgman@candle.pha.pa.us, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <12342.1019705420@sss.pgh.pa.us>
|
||
References: <Pine.NEB.4.43.0204251118040.445-100000@angelic.cynic.net>
|
||
<12342.1019705420@sss.pgh.pa.us>
|
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Message-ID: <20020425123429E.t-ishii@sra.co.jp>
|
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Date: Thu, 25 Apr 2002 12:34:29 +0900
|
||
From: Tatsuo Ishii <t-ishii@sra.co.jp>
|
||
X-Dispatcher: imput version 20000228(IM140)
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Status: OR
|
||
|
||
> Curt Sampson <cjs@cynic.net> writes:
|
||
> > Grabbing bigger chunks is always optimal, AFICT, if they're not
|
||
> > *too* big and you use the data. A single 64K read takes very little
|
||
> > longer than a single 8K read.
|
||
>
|
||
> Proof?
|
||
|
||
Long time ago I tested with the 32k block size and got 1.5-2x speed up
|
||
comparing ordinary 8k block size in the sequential scan case.
|
||
FYI, if this is the case.
|
||
--
|
||
Tatsuo Ishii
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
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TIP 5: Have you checked our extensive FAQ?
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|
||
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|
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From mloftis@wgops.com Thu Apr 25 01:43:14 2002
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Message-ID: <3CC7976F.7070407@wgops.com>
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Date: Wed, 24 Apr 2002 22:43:11 -0700
|
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
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cc: Curt Sampson <cjs@cynic.net>, Bruce Momjian <pgman@candle.pha.pa.us>,
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PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
References: <Pine.NEB.4.43.0204251118040.445-100000@angelic.cynic.net> <12342.1019705420@sss.pgh.pa.us>
|
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Status: OR
|
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|
||
|
||
|
||
Tom Lane wrote:
|
||
|
||
>Curt Sampson <cjs@cynic.net> writes:
|
||
>
|
||
>>Grabbing bigger chunks is always optimal, AFICT, if they're not
|
||
>>*too* big and you use the data. A single 64K read takes very little
|
||
>>longer than a single 8K read.
|
||
>>
|
||
>
|
||
>Proof?
|
||
>
|
||
I contend this statement.
|
||
|
||
It's optimal to a point. I know that my system settles into it's best
|
||
read-speeds @ 32K or 64K chunks. 8K chunks are far below optimal for my
|
||
system. Most systems I work on do far better at 16K than at 8K, and
|
||
most don't see any degradation when going to 32K chunks. (this is
|
||
across numerous OSes and configs -- results are interpretations from
|
||
bonnie disk i/o marks).
|
||
|
||
Depending on what you're doing it is more efficiend to read bigger
|
||
blocks up to a point. If you're multi-thread or reading in non-blocking
|
||
mode, take as big a chunk as you can handle or are ready to process in
|
||
quick order. If you're picking up a bunch of little chunks here and
|
||
there and know oyu're not using them again then choose a size that will
|
||
hopeuflly cause some of the reads to overlap, failing that, pick the
|
||
smallest usable read size.
|
||
|
||
The OS can never do that stuff for you.
|
||
|
||
|
||
|
||
From cjs@cynic.net Thu Apr 25 03:29:05 2002
|
||
Return-path: <cjs@cynic.net>
|
||
Received: from angelic.cynic.net ([202.232.117.21])
|
||
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|
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|
||
Received: from localhost (localhost [127.0.0.1])
|
||
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|
||
id 1C44E870E; Thu, 25 Apr 2002 16:28:51 +0900 (JST)
|
||
Date: Thu, 25 Apr 2002 16:28:51 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <12342.1019705420@sss.pgh.pa.us>
|
||
Message-ID: <Pine.NEB.4.43.0204251534590.3111-100000@angelic.cynic.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: OR
|
||
|
||
On Wed, 24 Apr 2002, Tom Lane wrote:
|
||
|
||
> Curt Sampson <cjs@cynic.net> writes:
|
||
> > Grabbing bigger chunks is always optimal, AFICT, if they're not
|
||
> > *too* big and you use the data. A single 64K read takes very little
|
||
> > longer than a single 8K read.
|
||
>
|
||
> Proof?
|
||
|
||
Well, there are various sorts of "proof" for this assertion. What
|
||
sort do you want?
|
||
|
||
Here's a few samples; if you're looking for something different to
|
||
satisfy you, let's discuss it.
|
||
|
||
1. Theoretical proof: two components of the delay in retrieving a
|
||
block from disk are the disk arm movement and the wait for the
|
||
right block to rotate under the head.
|
||
|
||
When retrieving, say, eight adjacent blocks, these will be spread
|
||
across no more than two cylinders (with luck, only one). The worst
|
||
case access time for a single block is the disk arm movement plus
|
||
the full rotational wait; this is the same as the worst case for
|
||
eight blocks if they're all on one cylinder. If they're not on one
|
||
cylinder, they're still on adjacent cylinders, requiring a very
|
||
short seek.
|
||
|
||
2. Proof by others using it: SQL server uses 64K reads when doing
|
||
table scans, as they say that their research indicates that the
|
||
major limitation is usually the number of I/O requests, not the
|
||
I/O capacity of the disk. BSD's explicitly separates the optimum
|
||
allocation size for storage (1K fragments) and optimum read size
|
||
(8K blocks) because they found performance to be much better when
|
||
a larger size block was read. Most file system vendors, too, do
|
||
read-ahead for this very reason.
|
||
|
||
3. Proof by testing. I wrote a little ruby program to seek to a
|
||
random point in the first 2 GB of my raw disk partition and read
|
||
1-8 8K blocks of data. (This was done as one I/O request.) (Using
|
||
the raw disk partition I avoid any filesystem buffering.) Here are
|
||
typical results:
|
||
|
||
125 reads of 16x8K blocks: 1.9 sec, 66.04 req/sec. 15.1 ms/req, 0.946 ms/block
|
||
250 reads of 8x8K blocks: 1.9 sec, 132.3 req/sec. 7.56 ms/req, 0.945 ms/block
|
||
500 reads of 4x8K blocks: 2.5 sec, 199 req/sec. 5.03 ms/req, 1.26 ms/block
|
||
1000 reads of 2x8K blocks: 3.8 sec, 261.6 req/sec. 3.82 ms/req, 1.91 ms/block
|
||
2000 reads of 1x8K blocks: 6.4 sec, 310.4 req/sec. 3.22 ms/req, 3.22 ms/block
|
||
|
||
The ratios of data retrieval speed per read for groups of adjacent
|
||
8K blocks, assuming a single 8K block reads in 1 time unit, are:
|
||
|
||
1 block 1.00
|
||
2 blocks 1.18
|
||
4 blocks 1.56
|
||
8 blocks 2.34
|
||
16 blocks 4.68
|
||
|
||
At less than 20% more expensive, certainly two-block read requests
|
||
could be considered to cost "very little more" than one-block read
|
||
requests. Even four-block read requests are only half-again as
|
||
expensive. And if you know you're really going to be using the
|
||
data, read in 8 block chunks and your cost per block (in terms of
|
||
time) drops to less than a third of the cost of single-block reads.
|
||
|
||
Let me put paid to comments about multiple simultaneous readers
|
||
making this invalid. Here's a typical result I get with four
|
||
instances of the program running simultaneously:
|
||
|
||
125 reads of 16x8K blocks: 4.4 sec, 28.21 req/sec. 35.4 ms/req, 2.22 ms/block
|
||
250 reads of 8x8K blocks: 3.9 sec, 64.88 req/sec. 15.4 ms/req, 1.93 ms/block
|
||
500 reads of 4x8K blocks: 5.8 sec, 86.52 req/sec. 11.6 ms/req, 2.89 ms/block
|
||
1000 reads of 2x8K blocks: 10 sec, 100.2 req/sec. 9.98 ms/req, 4.99 ms/block
|
||
2000 reads of 1x8K blocks: 18 sec, 110 req/sec. 9.09 ms/req, 9.09 ms/block
|
||
|
||
Here's the ratio table again, with another column comparing the
|
||
aggregate number of requests per second for one process and four
|
||
processes:
|
||
|
||
1 block 1.00 310 : 440
|
||
2 blocks 1.10 262 : 401
|
||
4 blocks 1.28 199 : 346
|
||
8 blocks 1.69 132 : 260
|
||
16 blocks 3.89 66 : 113
|
||
|
||
Note that, here the relative increase in performance for increasing
|
||
sizes of reads is even *better* until we get past 64K chunks. The
|
||
overall throughput is better, of course, because with more requests
|
||
per second coming in, the disk seek ordering code has more to work
|
||
with and the average seek time spent seeking vs. reading will be
|
||
reduced.
|
||
|
||
You know, this is not rocket science; I'm sure there must be papers
|
||
all over the place about this. If anybody still disagrees that it's
|
||
a good thing to read chunks up to 64K or so when the blocks are
|
||
adjacent and you know you'll need the data, I'd like to see some
|
||
tangible evidence to support that.
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
From cjs@cynic.net Thu Apr 25 03:55:59 2002
|
||
Return-path: <cjs@cynic.net>
|
||
Received: from angelic.cynic.net ([202.232.117.21])
|
||
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|
||
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|
||
Received: from localhost (localhost [127.0.0.1])
|
||
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|
||
id 188EC870E; Thu, 25 Apr 2002 16:55:51 +0900 (JST)
|
||
Date: Thu, 25 Apr 2002 16:55:50 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <200204250404.g3P44OI19061@candle.pha.pa.us>
|
||
Message-ID: <Pine.NEB.4.43.0204251636550.3111-100000@angelic.cynic.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: OR
|
||
|
||
On Thu, 25 Apr 2002, Bruce Momjian wrote:
|
||
|
||
> Well, we are guilty of trying to push as much as possible on to other
|
||
> software. We do this for portability reasons, and because we think our
|
||
> time is best spent dealing with db issues, not issues then can be deal
|
||
> with by other existing software, as long as the software is decent.
|
||
|
||
That's fine. I think that's a perfectly fair thing to do.
|
||
|
||
It was just the wording (i.e., "it's this other software's fault
|
||
that blah de blah") that got to me. To say, "We don't do readahead
|
||
becase most OSes supply it, and we feel that other things would
|
||
help more to improve performance," is fine by me. Or even, "Well,
|
||
nobody feels like doing it. You want it, do it yourself," I have
|
||
no problem with.
|
||
|
||
> Sure, that is certainly true. However, it is hard to know what the
|
||
> future will hold even if we had perfect knowledge of what was happening
|
||
> in the kernel. We don't know who else is going to start doing I/O once
|
||
> our I/O starts. We may have a better idea with kernel knowledge, but we
|
||
> still don't know 100% what will be cached.
|
||
|
||
Well, we do if we use raw devices and do our own caching, using
|
||
pages that are pinned in RAM. That was sort of what I was aiming
|
||
at for the long run.
|
||
|
||
> We have free-behind on our list.
|
||
|
||
Uh...can't do it, if you're relying on the OS to do the buffering.
|
||
How do you tell the OS that you're no longer going to use a page?
|
||
|
||
> I think LRU-K will do this quite well
|
||
> and be a nice general solution for more than just sequential scans.
|
||
|
||
LRU-K sounds like a great idea to me, as does putting pages read
|
||
for a table scan at the LRU end of the cache, rather than the MRU
|
||
(assuming we do something to ensure that they stay in cache until
|
||
read once, at any rate).
|
||
|
||
But again, great for your own cache, but doesn't work with the OS
|
||
cache. And I'm a bit scared to crank up too high the amount of
|
||
memory I give Postgres, lest the OS try to too aggressively buffer
|
||
all that I/O in what memory remains to it, and start blowing programs
|
||
(like maybe the backend binary itself) out of RAM. But maybe this
|
||
isn't typically a problem; I don't know.
|
||
|
||
> There may be validity in this. It is easy to do (I think) and could be
|
||
> a win.
|
||
|
||
It didn't look to difficult to me, when I looked at the code, and
|
||
you can see what kind of win it is from the response I just made
|
||
to Tom.
|
||
|
||
> > 1. It is *not* true that you have no idea where data is when
|
||
> > using a storage array or other similar system. While you
|
||
> > certainly ought not worry about things such as head positions
|
||
> > and so on, it's been a given for a long, long time that two
|
||
> > blocks that have close index numbers are going to be close
|
||
> > together in physical storage.
|
||
>
|
||
> SCSI drivers, for example, are pretty smart. Not sure we can take
|
||
> advantage of that from user-land I/O.
|
||
|
||
Looking at the NetBSD ones, I don't see what they're doing that's
|
||
so smart. (Aside from some awfully clever workarounds for stupid
|
||
hardware limitations that would otherwise kill performance.) What
|
||
sorts of "smart" are you referring to?
|
||
|
||
> Yes, but we are seeing some db's moving away from raw I/O.
|
||
|
||
Such as whom? And are you certain that they're moving to using the
|
||
OS buffer cache, too? MS SQL server, for example, uses the filesystem,
|
||
but turns off all buffering on those files.
|
||
|
||
> Our performance numbers beat most of the big db's already, so we must
|
||
> be doing something right.
|
||
|
||
Really? Do the performance numbers for simple, bulk operations
|
||
(imports, exports, table scans) beat the others handily? My intuition
|
||
says not, but I'll happily be convinced otherwise.
|
||
|
||
> Yes, but do we spend our time doing that. Is the payoff worth it, vs.
|
||
> working on other features. Sure it would be great to have all these
|
||
> fancy things, but is this where our time should be spent, considering
|
||
> other items on the TODO list?
|
||
|
||
I agree that these things need to be assesed.
|
||
|
||
> Jumping in and doing the I/O ourselves is a big undertaking, and looking
|
||
> at our TODO list, I am not sure if it is worth it right now.
|
||
|
||
Right. I'm not trying to say this is a critical priority, I'm just
|
||
trying to determine what we do right now, what we could do, and
|
||
the potential performance increase that would give us.
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
From cjs@cynic.net Thu Apr 25 05:19:11 2002
|
||
Return-path: <cjs@cynic.net>
|
||
Received: from angelic.cynic.net ([202.232.117.21])
|
||
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|
||
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|
||
Received: from localhost (localhost [127.0.0.1])
|
||
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|
||
id 50386870E; Thu, 25 Apr 2002 18:19:03 +0900 (JST)
|
||
Date: Thu, 25 Apr 2002 18:19:02 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <Pine.NEB.4.43.0204251534590.3111-100000@angelic.cynic.net>
|
||
Message-ID: <Pine.NEB.4.43.0204251805000.3111-100000@angelic.cynic.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: OR
|
||
|
||
On Thu, 25 Apr 2002, Curt Sampson wrote:
|
||
|
||
> Here's the ratio table again, with another column comparing the
|
||
> aggregate number of requests per second for one process and four
|
||
> processes:
|
||
>
|
||
|
||
Just for interest, I ran this again with 20 processes working
|
||
simultaneously. I did six runs at each blockread size and summed
|
||
the tps for each process to find the aggregate number of reads per
|
||
second during the test. I dropped the higest and the lowest ones,
|
||
and averaged the rest. Here's the new table:
|
||
|
||
1 proc 4 procs 20 procs
|
||
|
||
1 block 310 440 260
|
||
2 blocks 262 401 481
|
||
4 blocks 199 346 354
|
||
8 blocks 132 260 250
|
||
16 blocks 66 113 116
|
||
|
||
I'm not sure at all why performance gets so much *worse* with a lot of
|
||
contention on the 1K reads. This could have something to with NetBSD, or
|
||
its buffer cache, or my laptop's crappy little disk drive....
|
||
|
||
Or maybe I'm just running out of CPU.
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
From tgl@sss.pgh.pa.us Thu Apr 25 09:54:35 2002
|
||
Return-path: <tgl@sss.pgh.pa.us>
|
||
Received: from sss.pgh.pa.us (root@[192.204.191.242])
|
||
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|
||
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|
||
Received: from sss2.sss.pgh.pa.us (tgl@localhost [127.0.0.1])
|
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|
||
Thu, 25 Apr 2002 09:54:33 -0400 (EDT)
|
||
To: Curt Sampson <cjs@cynic.net>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <Pine.NEB.4.43.0204251534590.3111-100000@angelic.cynic.net>
|
||
References: <Pine.NEB.4.43.0204251534590.3111-100000@angelic.cynic.net>
|
||
Comments: In-reply-to Curt Sampson <cjs@cynic.net>
|
||
message dated "Thu, 25 Apr 2002 16:28:51 +0900"
|
||
Date: Thu, 25 Apr 2002 09:54:32 -0400
|
||
Message-ID: <25056.1019742872@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
Status: OR
|
||
|
||
Curt Sampson <cjs@cynic.net> writes:
|
||
> 1. Theoretical proof: two components of the delay in retrieving a
|
||
> block from disk are the disk arm movement and the wait for the
|
||
> right block to rotate under the head.
|
||
|
||
> When retrieving, say, eight adjacent blocks, these will be spread
|
||
> across no more than two cylinders (with luck, only one).
|
||
|
||
Weren't you contending earlier that with modern disk mechs you really
|
||
have no idea where the data is? You're asserting as an article of
|
||
faith that the OS has been able to place the file's data blocks
|
||
optimally --- or at least well enough to avoid unnecessary seeks.
|
||
But just a few days ago I was getting told that random_page_cost
|
||
was BS because there could be no such placement.
|
||
|
||
I'm getting a tad tired of sweeping generalizations offered without
|
||
proof, especially when they conflict.
|
||
|
||
> 3. Proof by testing. I wrote a little ruby program to seek to a
|
||
> random point in the first 2 GB of my raw disk partition and read
|
||
> 1-8 8K blocks of data. (This was done as one I/O request.) (Using
|
||
> the raw disk partition I avoid any filesystem buffering.)
|
||
|
||
And also ensure that you aren't testing the point at issue.
|
||
The point at issue is that *in the presence of kernel read-ahead*
|
||
it's quite unclear that there's any benefit to a larger request size.
|
||
Ideally the kernel will have the next block ready for you when you
|
||
ask, no matter what the request is.
|
||
|
||
There's been some talk of using the AIO interface (where available)
|
||
to "encourage" the kernel to do read-ahead. I don't foresee us
|
||
writing our own substitute filesystem to make this happen, however.
|
||
Oracle may have the manpower for that sort of boondoggle, but we
|
||
don't...
|
||
|
||
regards, tom lane
|
||
|
||
From pgsql-hackers-owner+M22053@postgresql.org Thu Apr 25 20:45:42 2002
|
||
Return-path: <pgsql-hackers-owner+M22053@postgresql.org>
|
||
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||
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|
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|
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|
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|
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|
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|
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Date: Thu, 25 Apr 2002 17:40:53 -0700
|
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To: PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <25056.1019742872@sss.pgh.pa.us>
|
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References: <Pine.NEB.4.43.0204251534590.3111-100000@angelic.cynic.net>
|
||
<25056.1019742872@sss.pgh.pa.us>
|
||
X-Mailer: VM 6.95 under 21.1 (patch 14) "Cuyahoga Valley" XEmacs Lucid
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
Status: ORr
|
||
|
||
Tom Lane wrote:
|
||
> ...
|
||
> Curt Sampson <cjs@cynic.net> writes:
|
||
> > 3. Proof by testing. I wrote a little ruby program to seek to a
|
||
> > random point in the first 2 GB of my raw disk partition and read
|
||
> > 1-8 8K blocks of data. (This was done as one I/O request.) (Using
|
||
> > the raw disk partition I avoid any filesystem buffering.)
|
||
>
|
||
> And also ensure that you aren't testing the point at issue.
|
||
> The point at issue is that *in the presence of kernel read-ahead*
|
||
> it's quite unclear that there's any benefit to a larger request size.
|
||
> Ideally the kernel will have the next block ready for you when you
|
||
> ask, no matter what the request is.
|
||
> ...
|
||
|
||
I have to agree with Tom. I think the numbers below show that with
|
||
kernel read-ahead, block size isn't an issue.
|
||
|
||
The big_file1 file used below is 2.0 gig of random data, and the
|
||
machine has 512 mb of main memory. This ensures that we're not
|
||
just getting cached data.
|
||
|
||
foreach i (4k 8k 16k 32k 64k 128k)
|
||
echo $i
|
||
time dd bs=$i if=big_file1 of=/dev/null
|
||
end
|
||
|
||
and the results:
|
||
|
||
bs user kernel elapsed
|
||
4k: 0.260 7.740 1:27.25
|
||
8k: 0.210 8.060 1:30.48
|
||
16k: 0.090 7.790 1:30.88
|
||
32k: 0.060 8.090 1:32.75
|
||
64k: 0.030 8.190 1:29.11
|
||
128k: 0.070 9.830 1:28.74
|
||
|
||
so with kernel read-ahead, we have basically the same elapsed (wall
|
||
time) regardless of block size. Sure, user time drops to a low at 64k
|
||
blocksize, but kernel time is increasing.
|
||
|
||
|
||
You could argue that this is a contrived example, no other I/O is
|
||
being done. Well I created a second 2.0g file (big_file2) and did two
|
||
simultaneous reads from the same disk. Sure performance went to hell
|
||
but it shows blocksize is still irrelevant in a multi I/O environment
|
||
with sequential read-ahead.
|
||
|
||
foreach i ( 4k 8k 16k 32k 64k 128k )
|
||
echo $i
|
||
time dd bs=$i if=big_file1 of=/dev/null &
|
||
time dd bs=$i if=big_file2 of=/dev/null &
|
||
wait
|
||
end
|
||
|
||
bs user kernel elapsed
|
||
4k: 0.480 8.290 6:34.13 bigfile1
|
||
0.320 8.730 6:34.33 bigfile2
|
||
8k: 0.250 7.580 6:31.75
|
||
0.180 8.450 6:31.88
|
||
16k: 0.150 8.390 6:32.47
|
||
0.100 7.900 6:32.55
|
||
32k: 0.190 8.460 6:24.72
|
||
0.060 8.410 6:24.73
|
||
64k: 0.060 9.350 6:25.05
|
||
0.150 9.240 6:25.13
|
||
128k: 0.090 10.610 6:33.14
|
||
0.110 11.320 6:33.31
|
||
|
||
|
||
the differences in read times are basically in the mud. Blocksize
|
||
just doesn't matter much with the kernel doing readahead.
|
||
|
||
-Kyle
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 6: Have you searched our list archives?
|
||
|
||
http://archives.postgresql.org
|
||
|
||
From pgsql-hackers-owner+M22055@postgresql.org Thu Apr 25 22:19:07 2002
|
||
Return-path: <pgsql-hackers-owner+M22055@postgresql.org>
|
||
Received: from postgresql.org (postgresql.org [64.49.215.8])
|
||
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|
||
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|
||
Received: from postgresql.org (postgresql.org [64.49.215.8])
|
||
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|
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|
||
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|
||
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|
||
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|
||
Received: (from pgman@localhost)
|
||
by candle.pha.pa.us (8.11.6/8.10.1) id g3Q2Ili11246;
|
||
Thu, 25 Apr 2002 22:18:47 -0400 (EDT)
|
||
From: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Message-ID: <200204260218.g3Q2Ili11246@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <15560.41493.529847.635632@doppelbock.patentinvestor.com>
|
||
To: Kyle <kaf@nwlink.com>
|
||
Date: Thu, 25 Apr 2002 22:18:47 -0400 (EDT)
|
||
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
X-Mailer: ELM [version 2.4ME+ PL97 (25)]
|
||
MIME-Version: 1.0
|
||
Content-Transfer-Encoding: 7bit
|
||
Content-Type: text/plain; charset=US-ASCII
|
||
Precedence: bulk
|
||
Sender: pgsql-hackers-owner@postgresql.org
|
||
Status: OR
|
||
|
||
|
||
Nice test. Would you test simultaneous 'dd' on the same file, perhaps
|
||
with a slight delay between to the two so they don't read each other's
|
||
blocks?
|
||
|
||
seek() in the file will turn off read-ahead in most OS's. I am not
|
||
saying this is a major issue for PostgreSQL but the numbers would be
|
||
interesting.
|
||
|
||
|
||
---------------------------------------------------------------------------
|
||
|
||
Kyle wrote:
|
||
> Tom Lane wrote:
|
||
> > ...
|
||
> > Curt Sampson <cjs@cynic.net> writes:
|
||
> > > 3. Proof by testing. I wrote a little ruby program to seek to a
|
||
> > > random point in the first 2 GB of my raw disk partition and read
|
||
> > > 1-8 8K blocks of data. (This was done as one I/O request.) (Using
|
||
> > > the raw disk partition I avoid any filesystem buffering.)
|
||
> >
|
||
> > And also ensure that you aren't testing the point at issue.
|
||
> > The point at issue is that *in the presence of kernel read-ahead*
|
||
> > it's quite unclear that there's any benefit to a larger request size.
|
||
> > Ideally the kernel will have the next block ready for you when you
|
||
> > ask, no matter what the request is.
|
||
> > ...
|
||
>
|
||
> I have to agree with Tom. I think the numbers below show that with
|
||
> kernel read-ahead, block size isn't an issue.
|
||
>
|
||
> The big_file1 file used below is 2.0 gig of random data, and the
|
||
> machine has 512 mb of main memory. This ensures that we're not
|
||
> just getting cached data.
|
||
>
|
||
> foreach i (4k 8k 16k 32k 64k 128k)
|
||
> echo $i
|
||
> time dd bs=$i if=big_file1 of=/dev/null
|
||
> end
|
||
>
|
||
> and the results:
|
||
>
|
||
> bs user kernel elapsed
|
||
> 4k: 0.260 7.740 1:27.25
|
||
> 8k: 0.210 8.060 1:30.48
|
||
> 16k: 0.090 7.790 1:30.88
|
||
> 32k: 0.060 8.090 1:32.75
|
||
> 64k: 0.030 8.190 1:29.11
|
||
> 128k: 0.070 9.830 1:28.74
|
||
>
|
||
> so with kernel read-ahead, we have basically the same elapsed (wall
|
||
> time) regardless of block size. Sure, user time drops to a low at 64k
|
||
> blocksize, but kernel time is increasing.
|
||
>
|
||
>
|
||
> You could argue that this is a contrived example, no other I/O is
|
||
> being done. Well I created a second 2.0g file (big_file2) and did two
|
||
> simultaneous reads from the same disk. Sure performance went to hell
|
||
> but it shows blocksize is still irrelevant in a multi I/O environment
|
||
> with sequential read-ahead.
|
||
>
|
||
> foreach i ( 4k 8k 16k 32k 64k 128k )
|
||
> echo $i
|
||
> time dd bs=$i if=big_file1 of=/dev/null &
|
||
> time dd bs=$i if=big_file2 of=/dev/null &
|
||
> wait
|
||
> end
|
||
>
|
||
> bs user kernel elapsed
|
||
> 4k: 0.480 8.290 6:34.13 bigfile1
|
||
> 0.320 8.730 6:34.33 bigfile2
|
||
> 8k: 0.250 7.580 6:31.75
|
||
> 0.180 8.450 6:31.88
|
||
> 16k: 0.150 8.390 6:32.47
|
||
> 0.100 7.900 6:32.55
|
||
> 32k: 0.190 8.460 6:24.72
|
||
> 0.060 8.410 6:24.73
|
||
> 64k: 0.060 9.350 6:25.05
|
||
> 0.150 9.240 6:25.13
|
||
> 128k: 0.090 10.610 6:33.14
|
||
> 0.110 11.320 6:33.31
|
||
>
|
||
>
|
||
> the differences in read times are basically in the mud. Blocksize
|
||
> just doesn't matter much with the kernel doing readahead.
|
||
>
|
||
> -Kyle
|
||
>
|
||
> ---------------------------(end of broadcast)---------------------------
|
||
> TIP 6: Have you searched our list archives?
|
||
>
|
||
> http://archives.postgresql.org
|
||
>
|
||
|
||
--
|
||
Bruce Momjian | http://candle.pha.pa.us
|
||
pgman@candle.pha.pa.us | (610) 853-3000
|
||
+ If your life is a hard drive, | 830 Blythe Avenue
|
||
+ Christ can be your backup. | Drexel Hill, Pennsylvania 19026
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 6: Have you searched our list archives?
|
||
|
||
http://archives.postgresql.org
|
||
|
||
From cjs@cynic.net Thu Apr 25 22:27:23 2002
|
||
Return-path: <cjs@cynic.net>
|
||
Received: from angelic.cynic.net ([202.232.117.21])
|
||
by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id g3Q2RL411868
|
||
for <pgman@candle.pha.pa.us>; Thu, 25 Apr 2002 22:27:22 -0400 (EDT)
|
||
Received: from localhost (localhost [127.0.0.1])
|
||
by angelic.cynic.net (Postfix) with ESMTP
|
||
id AF60C870E; Fri, 26 Apr 2002 11:27:17 +0900 (JST)
|
||
Date: Fri, 26 Apr 2002 11:27:17 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <25056.1019742872@sss.pgh.pa.us>
|
||
Message-ID: <Pine.NEB.4.43.0204261028110.449-100000@angelic.cynic.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: OR
|
||
|
||
On Thu, 25 Apr 2002, Tom Lane wrote:
|
||
|
||
> Curt Sampson <cjs@cynic.net> writes:
|
||
> > 1. Theoretical proof: two components of the delay in retrieving a
|
||
> > block from disk are the disk arm movement and the wait for the
|
||
> > right block to rotate under the head.
|
||
>
|
||
> > When retrieving, say, eight adjacent blocks, these will be spread
|
||
> > across no more than two cylinders (with luck, only one).
|
||
>
|
||
> Weren't you contending earlier that with modern disk mechs you really
|
||
> have no idea where the data is?
|
||
|
||
No, that was someone else. I contend that with pretty much any
|
||
large-scale storage mechanism (i.e., anything beyond ramdisks),
|
||
you will find that accessing two adjacent blocks is almost always
|
||
1) close to as fast as accessing just the one, and 2) much, much
|
||
faster than accessing two blocks that are relatively far apart.
|
||
|
||
There will be the odd case where the two adjacent blocks are
|
||
physically far apart, but this is rare.
|
||
|
||
If this idea doesn't hold true, the whole idea that sequential
|
||
reads are faster than random reads falls apart, and the optimizer
|
||
shouldn't even have the option to make random reads cost more, much
|
||
less have it set to four rather than one (or whatever it's set to).
|
||
|
||
> You're asserting as an article of
|
||
> faith that the OS has been able to place the file's data blocks
|
||
> optimally --- or at least well enough to avoid unnecessary seeks.
|
||
|
||
So are you, in the optimizer. But that's all right; the OS often
|
||
can and does do this placement; the FFS filesystem is explicitly
|
||
designed to do this sort of thing. If the filesystem isn't empty
|
||
and the files grow a lot they'll be split into large fragments,
|
||
but the fragments will be contiguous.
|
||
|
||
> But just a few days ago I was getting told that random_page_cost
|
||
> was BS because there could be no such placement.
|
||
|
||
I've been arguing against that point as well.
|
||
|
||
> And also ensure that you aren't testing the point at issue.
|
||
> The point at issue is that *in the presence of kernel read-ahead*
|
||
> it's quite unclear that there's any benefit to a larger request size.
|
||
|
||
I will test this.
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
From cjs@cynic.net Wed Apr 24 23:19:23 2002
|
||
Return-path: <cjs@cynic.net>
|
||
Received: from angelic.cynic.net ([202.232.117.21])
|
||
by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id g3P3JM414917
|
||
for <pgman@candle.pha.pa.us>; Wed, 24 Apr 2002 23:19:22 -0400 (EDT)
|
||
Received: from localhost (localhost [127.0.0.1])
|
||
by angelic.cynic.net (Postfix) with ESMTP
|
||
id 1F36F870E; Thu, 25 Apr 2002 12:19:14 +0900 (JST)
|
||
Date: Thu, 25 Apr 2002 12:19:14 +0900 (JST)
|
||
From: Curt Sampson <cjs@cynic.net>
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
cc: PostgreSQL-development <pgsql-hackers@postgresql.org>
|
||
Subject: Re: Sequential Scan Read-Ahead
|
||
In-Reply-To: <200204250156.g3P1ufh05751@candle.pha.pa.us>
|
||
Message-ID: <Pine.NEB.4.43.0204251118040.445-100000@angelic.cynic.net>
|
||
MIME-Version: 1.0
|
||
Content-Type: TEXT/PLAIN; charset=US-ASCII
|
||
Status: OR
|
||
|
||
On Wed, 24 Apr 2002, Bruce Momjian wrote:
|
||
|
||
> > 1. Not all systems do readahead.
|
||
>
|
||
> If they don't, that isn't our problem. We expect it to be there, and if
|
||
> it isn't, the vendor/kernel is at fault.
|
||
|
||
It is your problem when another database kicks Postgres' ass
|
||
performance-wise.
|
||
|
||
And at that point, *you're* at fault. You're the one who's knowingly
|
||
decided to do things inefficiently.
|
||
|
||
Sorry if this sounds harsh, but this, "Oh, someone else is to blame"
|
||
attitude gets me steamed. It's one thing to say, "We don't support
|
||
this." That's fine; there are often good reasons for that. It's a
|
||
completely different thing to say, "It's an unrelated entity's fault we
|
||
don't support this."
|
||
|
||
At any rate, relying on the kernel to guess how to optimise for
|
||
the workload will never work as well as well as the software that
|
||
knows the workload doing the optimization.
|
||
|
||
The lack of support thing is no joke. Sure, lots of systems nowadays
|
||
support unified buffer cache and read-ahead. But how many, besides
|
||
Solaris, support free-behind, which is also very important to avoid
|
||
blowing out your buffer cache when doing sequential reads? And who
|
||
at all supports read-ahead for reverse scans? (Or does Postgres
|
||
not do those, anyway? I can see the support is there.)
|
||
|
||
And even when the facilities are there, you create problems by
|
||
using them. Look at the OS buffer cache, for example. Not only do
|
||
we lose efficiency by using two layers of caching, but (as people
|
||
have pointed out recently on the lists), the optimizer can't even
|
||
know how much or what is being cached, and thus can't make decisions
|
||
based on that.
|
||
|
||
> Yes, seek() in file will turn off read-ahead. Grabbing bigger chunks
|
||
> would help here, but if you have two people already reading from the
|
||
> same file, grabbing bigger chunks of the file may not be optimal.
|
||
|
||
Grabbing bigger chunks is always optimal, AFICT, if they're not
|
||
*too* big and you use the data. A single 64K read takes very little
|
||
longer than a single 8K read.
|
||
|
||
> > 3. Even when the read-ahead does occur, you're still doing more
|
||
> > syscalls, and thus more expensive kernel/userland transitions, than
|
||
> > you have to.
|
||
>
|
||
> I would guess the performance impact is minimal.
|
||
|
||
If it were minimal, people wouldn't work so hard to build multi-level
|
||
thread systems, where multiple userland threads are scheduled on
|
||
top of kernel threads.
|
||
|
||
However, it does depend on how much CPU your particular application
|
||
is using. You may have it to spare.
|
||
|
||
> http://candle.pha.pa.us/mhonarc/todo.detail/performance/msg00009.html
|
||
|
||
Well, this message has some points in it that I feel are just incorrect.
|
||
|
||
1. It is *not* true that you have no idea where data is when
|
||
using a storage array or other similar system. While you
|
||
certainly ought not worry about things such as head positions
|
||
and so on, it's been a given for a long, long time that two
|
||
blocks that have close index numbers are going to be close
|
||
together in physical storage.
|
||
|
||
2. Raw devices are quite standard across Unix systems (except
|
||
in the unfortunate case of Linux, which I think has been
|
||
remedied, hasn't it?). They're very portable, and have just as
|
||
well--if not better--defined write semantics as a filesystem.
|
||
|
||
3. My observations of OS performance tuning over the past six
|
||
or eight years contradict the statement, "There's a considerable
|
||
cost in complexity and code in using "raw" storage too, and
|
||
it's not a one off cost: as the technologies change, the "fast"
|
||
way to do things will change and the code will have to be
|
||
updated to match." While optimizations have been removed over
|
||
the years the basic optimizations (order reads by block number,
|
||
do larger reads rather than smaller, cache the data) have
|
||
remained unchanged for a long, long time.
|
||
|
||
4. "Better to leave this to the OS vendor where possible, and
|
||
take advantage of the tuning they do." Well, sorry guys, but
|
||
have a look at the tuning they do. It hasn't changed in years,
|
||
except to remove now-unnecessary complexity realated to really,
|
||
really old and slow disk devices, and to add a few thing that
|
||
guess workload but still do a worse job than if the workload
|
||
generator just did its own optimisations in the first place.
|
||
|
||
> http://candle.pha.pa.us/mhonarc/todo.detail/optimizer/msg00011.html
|
||
|
||
Well, this one, with statements like "Postgres does have control
|
||
over its buffer cache," I don't know what to say. You can interpret
|
||
the statement however you like, but in the end Postgres very little
|
||
control at all over how data is moved between memory and disk.
|
||
|
||
BTW, please don't take me as saying that all control over physical
|
||
IO should be done by Postgres. I just think that Posgres could do
|
||
a better job of managing data transfer between disk and memory than
|
||
the OS can. The rest of the things (using raw paritions, read-ahead,
|
||
free-behind, etc.) just drop out of that one idea.
|
||
|
||
cjs
|
||
--
|
||
Curt Sampson <cjs@cynic.net> +81 90 7737 2974 http://www.netbsd.org
|
||
Don't you know, in this new Dark Age, we're all light. --XTC
|
||
|
||
|
||
From kaf@nwlink.com Fri Apr 26 14:22:39 2002
|
||
Return-path: <kaf@nwlink.com>
|
||
Received: from doppelbock.patentinvestor.com (ip146.usw5.rb1.bel.nwlink.com [209.20.249.146])
|
||
by candle.pha.pa.us (8.11.6/8.10.1) with ESMTP id g3QIMc400783
|
||
for <pgman@candle.pha.pa.us>; Fri, 26 Apr 2002 14:22:38 -0400 (EDT)
|
||
Received: (from kaf@localhost)
|
||
by doppelbock.patentinvestor.com (8.11.6/8.11.2) id g3QII0l16824;
|
||
Fri, 26 Apr 2002 11:18:00 -0700
|
||
From: Kyle <kaf@nwlink.com>
|
||
MIME-Version: 1.0
|
||
Content-Type: text/plain; charset=us-ascii
|
||
Content-Transfer-Encoding: 7bit
|
||
Message-ID: <15561.39384.296503.501888@doppelbock.patentinvestor.com>
|
||
Date: Fri, 26 Apr 2002 11:18:00 -0700
|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Sequential Scan Read-Ahead
|
||
In-Reply-To: <200204261444.g3QEiFh11090@candle.pha.pa.us>
|
||
References: <15561.26116.817541.950416@doppelbock.patentinvestor.com>
|
||
<200204261444.g3QEiFh11090@candle.pha.pa.us>
|
||
X-Mailer: VM 6.95 under 21.1 (patch 14) "Cuyahoga Valley" XEmacs Lucid
|
||
Status: ORr
|
||
|
||
Hey Bruce,
|
||
|
||
I'll forward this to the list if you think they'd benefit from it.
|
||
I'm not sure it says anything about read-ahead, I think this is more a
|
||
kernel caching issue. But I've been known to be wrong in the past.
|
||
Anyway...
|
||
|
||
|
||
the test:
|
||
|
||
foreach i (5 15 20 25 30 )
|
||
echo $i
|
||
time dd bs=8k if=big_file1 of=/dev/null &
|
||
sleep $i
|
||
time dd bs=8k if=big_file1 of=/dev/null &
|
||
wait
|
||
end
|
||
|
||
I did a couple more runs in the low range since their is a drastic
|
||
jump in elapsed (wall clock) time after doing a 6 second sleep:
|
||
|
||
first process second process
|
||
sleep user kernel elapsed user kernel elapsed
|
||
0 sec 0.200 7.980 1:26.57 0.240 7.720 1:26.56
|
||
3 sec 0.260 7.600 1:25.71 0.260 8.100 1:22.60
|
||
5 sec 0.160 7.890 1:26.04 0.220 8.180 1:21.04
|
||
6 sec 0.220 8.070 1:19.59 0.230 7.620 1:25.69
|
||
7 sec 0.210 9.270 1:57.92 0.100 8.750 1:50.76
|
||
8 sec 0.240 8.060 4:47.47 0.300 7.800 4:40.40
|
||
15 sec 0.200 8.500 4:51.11 0.180 7.280 4:44.36
|
||
20 sec 0.160 8.040 4:40.72 0.240 7.790 4:37.24
|
||
25 sec 0.170 8.150 4:37.58 0.140 8.200 4:33.08
|
||
30 sec 0.200 7.390 4:37.01 0.230 8.220 4:31.83
|
||
|
||
|
||
|
||
with a sleep of > 6 seconds, either the second process isn't getting
|
||
cached data or readahead is being turned off. I'd guess the former, I
|
||
don't see why read-ahead would be turned off since they're both doing
|
||
sequential operations.
|
||
|
||
Although with 512mb of memory and the disk reading at about 22 mb/sec,
|
||
maybe we're not hitting the cache. I'd guess at least ~400 megs of
|
||
kernel cache is being used for buffering this 2 gig file. free(1)
|
||
reports:
|
||
|
||
% free
|
||
total used free shared buffers cached
|
||
Mem: 512924 508576 4348 0 2640 477960
|
||
-/+ buffers/cache: 27976 484948
|
||
Swap: 527152 15864 511288
|
||
|
||
so shouldn't we be getting cached data even with a sleep of up to
|
||
about (400/22) 18 seconds...? Maybe I'm just in the dark on what's
|
||
really happening. I should point out that this is linux 2.4.18.
|
||
|
||
|
||
|
||
|
||
Bruce Momjian wrote:
|
||
>
|
||
> I am trying to illustrate how kernel read-ahead could be turned off in
|
||
> certain cases.
|
||
>
|
||
> ---------------------------------------------------------------------------
|
||
>
|
||
> Kyle wrote:
|
||
> > What are you trying to test, the kernel's cache vs disk speed?
|
||
> >
|
||
> >
|
||
> > Bruce Momjian wrote:
|
||
> > >
|
||
> > > Nice test. Would you test simultaneous 'dd' on the same file, perhaps
|
||
> > > with a slight delay between to the two so they don't read each other's
|
||
> > > blocks?
|
||
> > >
|
||
> > > seek() in the file will turn off read-ahead in most OS's. I am not
|
||
> > > saying this is a major issue for PostgreSQL but the numbers would be
|
||
> > > interesting.
|
||
|
||
From pgsql-hackers-owner+M49418=pgman=candle.pha.pa.us@postgresql.org Tue Jan 27 15:52:28 2004
|
||
Return-path: <pgsql-hackers-owner+M49418=pgman=candle.pha.pa.us@postgresql.org>
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||
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|
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|
||
To: pgsql-hackers@postgresql.org
|
||
Subject: [HACKERS] Question about indexes
|
||
From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
|
||
Date: 27 Jan 2004 15:36:20 -0500
|
||
Message-ID: <87isixt9h7.fsf@stark.xeocode.com>
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version=2.61
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Status: OR
|
||
|
||
|
||
How feasible would it be to have a btree index on ctid? I'm thinking it ought
|
||
to work simply enough for the normal case of insert/delet/update, but I'm not
|
||
completely certain how vacuum, vacuum full, and cluster would interact.
|
||
|
||
You may think this would be utterly useless, but I have a cunning plan.
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 8: explain analyze is your friend
|
||
|
||
From pgsql-hackers-owner+M49439=pgman=candle.pha.pa.us@postgresql.org Tue Jan 27 18:01:59 2004
|
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Tue, 27 Jan 2004 17:51:56 -0500 (EST)
|
||
To: Greg Stark <gsstark@mit.edu>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <87isixt9h7.fsf@stark.xeocode.com>
|
||
References: <87isixt9h7.fsf@stark.xeocode.com>
|
||
Comments: In-reply-to Greg Stark <gsstark@mit.edu>
|
||
message dated "27 Jan 2004 15:36:20 -0500"
|
||
Date: Tue, 27 Jan 2004 17:51:56 -0500
|
||
Message-ID: <29815.1075243916@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by amavisd-new at postgresql.org
|
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|
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version=2.61
|
||
Status: OR
|
||
|
||
Greg Stark <gsstark@mit.edu> writes:
|
||
> How feasible would it be to have a btree index on ctid?
|
||
|
||
Why would you want one? Direct access by ctid beats out an index lookup
|
||
every time. In any case, vacuum and friends would break such an index
|
||
entirely.
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 3: if posting/reading through Usenet, please send an appropriate
|
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subscribe-nomail command to majordomo@postgresql.org so that your
|
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From pgsql-hackers-owner+M49440=pgman=candle.pha.pa.us@postgresql.org Tue Jan 27 18:19:13 2004
|
||
Return-path: <pgsql-hackers-owner+M49440=pgman=candle.pha.pa.us@postgresql.org>
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
References: <87isixt9h7.fsf@stark.xeocode.com>
|
||
<29815.1075243916@sss.pgh.pa.us>
|
||
In-Reply-To: <29815.1075243916@sss.pgh.pa.us>
|
||
From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
|
||
Date: 27 Jan 2004 18:11:31 -0500
|
||
Message-ID: <87d695t2ak.fsf@stark.xeocode.com>
|
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version=2.61
|
||
Status: OR
|
||
|
||
Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
|
||
> Greg Stark <gsstark@mit.edu> writes:
|
||
>
|
||
> > How feasible would it be to have a btree index on ctid?
|
||
>
|
||
> Why would you want one? Direct access by ctid beats out an index lookup
|
||
> every time.
|
||
|
||
Of course. But as I mentioned, I have a cunning plan.
|
||
|
||
If you have two indexes (a,ctid) and (b,ctid) and do a query where a=1 and b=2
|
||
then it would be particularly easy to combine the two efficiently.
|
||
|
||
If specially marked btree indexes -- or even all btree indexes -- implicitly
|
||
had ctid as a final sort order after all the index column, then it would
|
||
esentially obviate the need for bitmap indexes. They wouldn't have the space
|
||
advantage, but they would be possible to combine using arbitrary boolean
|
||
expressions without looking at the actual tuples.
|
||
|
||
This is essentially what is in the TODO about using bitmaps, but without
|
||
having to do any extra sorts.
|
||
|
||
This would only really be an advantage for particularly wide tables where the
|
||
combination of boolean clauses narrows the result set down a lot more than any
|
||
one clause.
|
||
|
||
> In any case, vacuum and friends would break such an index entirely.
|
||
|
||
That was what I was afraid of.
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 5: Have you checked our extensive FAQ?
|
||
|
||
http://www.postgresql.org/docs/faqs/FAQ.html
|
||
|
||
From pgsql-hackers-owner+M49442=pgman=candle.pha.pa.us@postgresql.org Tue Jan 27 18:32:25 2004
|
||
Return-path: <pgsql-hackers-owner+M49442=pgman=candle.pha.pa.us@postgresql.org>
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||
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|
||
To: Greg Stark <gsstark@mit.edu>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <87d695t2ak.fsf@stark.xeocode.com>
|
||
References: <87isixt9h7.fsf@stark.xeocode.com> <29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com>
|
||
Comments: In-reply-to Greg Stark <gsstark@mit.edu>
|
||
message dated "27 Jan 2004 18:11:31 -0500"
|
||
Date: Tue, 27 Jan 2004 18:24:41 -0500
|
||
Message-ID: <403.1075245881@sss.pgh.pa.us>
|
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candle.pha.pa.us
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X-Spam-Status: No, hits=-4.9 required=5.0 tests=BAYES_00 autolearn=ham
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version=2.61
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Status: OR
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||
|
||
Greg Stark <gsstark@mit.edu> writes:
|
||
> If you have two indexes (a,ctid) and (b,ctid) and do a query where a=1 and b=2
|
||
> then it would be particularly easy to combine the two efficiently.
|
||
|
||
> If specially marked btree indexes -- or even all btree indexes -- implicitly
|
||
> had ctid as a final sort order after all the index column, then it would
|
||
> esentially obviate the need for bitmap indexes.
|
||
|
||
I don't think so. You are thinking only of exact-equality queries ---
|
||
as soon as the WHERE clause describes a range of index entries, the
|
||
readout wouldn't be sorted by ctid anyway.
|
||
|
||
Combining indexes via a bitmap intermediate step (which is not really
|
||
the same thing as bitmap indexes, IIUC) seems like a more robust
|
||
approach than relying on the index entries to be in ctid order.
|
||
|
||
But if we did want to sort indexes that way, we could do it today,
|
||
I think. The ctid is already stored in index entries (it is the
|
||
"payload" remember...) and we could use it as a tiebreaker when
|
||
determining insertion position. This doesn't have the problems that
|
||
putting ctid into the user columns would do, because the system knows
|
||
about that ctid as being special; the difficulty with ctid in the user
|
||
columns is the code not knowing that it'd need to change on a tuple move.
|
||
|
||
regards, tom lane
|
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
References: <87isixt9h7.fsf@stark.xeocode.com>
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<29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com>
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<403.1075245881@sss.pgh.pa.us>
|
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In-Reply-To: <403.1075245881@sss.pgh.pa.us>
|
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From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
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Date: 27 Jan 2004 21:19:26 -0500
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Message-ID: <877jzcu85t.fsf@stark.xeocode.com>
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|
||
Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
|
||
> I don't think so. You are thinking only of exact-equality queries ---
|
||
> as soon as the WHERE clause describes a range of index entries, the
|
||
> readout wouldn't be sorted by ctid anyway.
|
||
|
||
But then even bitmap indexes would fail in that way too, or at least have a
|
||
lot of extra cost that would have to be taken into account based on the number
|
||
of values in the range.
|
||
|
||
> Combining indexes via a bitmap intermediate step (which is not really
|
||
> the same thing as bitmap indexes, IIUC) seems like a more robust
|
||
> approach than relying on the index entries to be in ctid order.
|
||
|
||
I would see that as the next step, But it seems to me it would be only a small
|
||
set of queries where it would really help enough to outweigh the extra work of
|
||
the sort. Whereas if the ctid is already pre-sorted then the extra cost is
|
||
fairly low. Sort of like the difference in cost between a merge join where
|
||
both sides have to be sorted and a merge join where both sides are pre-sorted.
|
||
|
||
> But if we did want to sort indexes that way, we could do it today,
|
||
> I think. The ctid is already stored in index entries (it is the
|
||
> "payload" remember...) and we could use it as a tiebreaker when
|
||
> determining insertion position. This doesn't have the problems that
|
||
> putting ctid into the user columns would do, because the system knows
|
||
> about that ctid as being special; the difficulty with ctid in the user
|
||
> columns is the code not knowing that it'd need to change on a tuple move.
|
||
|
||
That's exactly what I was thinking. I just don't know how badly it would
|
||
complicate the vacuum{,full}/cluster code and whether those are the only cases
|
||
to worry about.
|
||
|
||
|
||
Note that the space saving of bitmap indexes is still a substantial factor.
|
||
Using btree indexes the i/o costs of doing multiple index scans plus a table
|
||
scan of the relevant pages would still be quite substantial. So this doesn't
|
||
completely obviate the need for bitmap indexes, but I think it would remove a
|
||
lot of the pressure from people who just need them to handle a few select
|
||
queries.
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
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||
TIP 1: subscribe and unsubscribe commands go to majordomo@postgresql.org
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||
|
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From pgsql-hackers-owner+M49453=pgman=candle.pha.pa.us@postgresql.org Tue Jan 27 21:53:09 2004
|
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Return-path: <pgsql-hackers-owner+M49453=pgman=candle.pha.pa.us@postgresql.org>
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To: Greg Stark <gsstark@mit.edu>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <877jzcu85t.fsf@stark.xeocode.com>
|
||
References: <87isixt9h7.fsf@stark.xeocode.com> <29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com> <403.1075245881@sss.pgh.pa.us> <877jzcu85t.fsf@stark.xeocode.com>
|
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Comments: In-reply-to Greg Stark <gsstark@mit.edu>
|
||
message dated "27 Jan 2004 21:19:26 -0500"
|
||
Date: Tue, 27 Jan 2004 21:48:59 -0500
|
||
Message-ID: <5813.1075258139@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by amavisd-new at postgresql.org
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|
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version=2.61
|
||
Status: OR
|
||
|
||
Greg Stark <gsstark@mit.edu> writes:
|
||
>> Combining indexes via a bitmap intermediate step (which is not really
|
||
>> the same thing as bitmap indexes, IIUC) seems like a more robust
|
||
>> approach than relying on the index entries to be in ctid order.
|
||
|
||
> I would see that as the next step, But it seems to me it would be only a small
|
||
> set of queries where it would really help enough to outweigh the extra work of
|
||
> the sort.
|
||
|
||
What sort? The whole point of a bitmap is that it makes it easy to
|
||
visit the tuples in heap order. You scan the index, you set the
|
||
appropriate bits in the bitmap, and then you scan the bitmap and go to
|
||
the heap tuples that have their bits set. If you are using multiple
|
||
indexes you can AND or OR their results at the bitmap phase before you
|
||
go to the heap.
|
||
|
||
An implementation of this kind would not produce tuples in index order,
|
||
so if you have an ORDER BY to satisfy then you end up doing an explicit
|
||
sort after you have the tuples. It would be up to the planner to
|
||
consider this cost versus the advantages of being able to use multiple
|
||
indexes; we'd certainly want to keep the existing scan mechanism as an
|
||
available alternative. But if the query is suited to multiple indexes
|
||
I suspect it'd be a win pretty often.
|
||
|
||
> Note that the space saving of bitmap indexes is still a substantial factor.
|
||
|
||
I think you are still confusing what I'm talking about with a bitmap
|
||
index, ie, a persistent structure on-disk. It's not that at all, but
|
||
a transient structure built in-memory during an index scan.
|
||
|
||
I'm a little dubious that true bitmap indexes would be worth building
|
||
for Postgres. Seems like partial indexes cover the same sorts of
|
||
applications and are more flexible.
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
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From pgsql-hackers-owner+M49462=pgman=candle.pha.pa.us@postgresql.org Wed Jan 28 13:10:48 2004
|
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Return-path: <pgsql-hackers-owner+M49462=pgman=candle.pha.pa.us@postgresql.org>
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
References: <87isixt9h7.fsf@stark.xeocode.com>
|
||
<29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com>
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<403.1075245881@sss.pgh.pa.us> <877jzcu85t.fsf@stark.xeocode.com>
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<5813.1075258139@sss.pgh.pa.us>
|
||
In-Reply-To: <5813.1075258139@sss.pgh.pa.us>
|
||
From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
|
||
Date: 28 Jan 2004 09:13:47 -0500
|
||
Message-ID: <871xpktb38.fsf@stark.xeocode.com>
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version=2.61
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|
||
Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
|
||
> Greg Stark <gsstark@mit.edu> writes:
|
||
> >
|
||
> > I would see that as the next step, But it seems to me it would be only a small
|
||
> > set of queries where it would really help enough to outweigh the extra work of
|
||
> > the sort.
|
||
>
|
||
> What sort?
|
||
|
||
To build the in-memory bitmap you effectively have to do a sort. If the tuples
|
||
come out of the index in heap order then you can combine them without having
|
||
to go through that step.
|
||
|
||
> I'm a little dubious that true bitmap indexes would be worth building
|
||
> for Postgres. Seems like partial indexes cover the same sorts of
|
||
> applications and are more flexible.
|
||
|
||
I'm clear on the distinction. I think bitmap indexes still have a place, but
|
||
if regular btree indexes could be combined efficiently then that would be an
|
||
even narrower niche.
|
||
|
||
Partial indexes are very handy, and they're useful in corner cases where
|
||
bitmap indexes are useful, such as flags for special types of records.
|
||
|
||
But I think bitmap indexes are specifically wanted by certain types of data
|
||
warehousing applications where you have an index on virtually every column and
|
||
then want to do arbitrary boolean combinations of all of them. btree indexes
|
||
would generate more i/o scanning all the indexes than just doing a sequential
|
||
scan would. Whereas bitmap indexes are much denser on disk.
|
||
|
||
However my experience leans more towards the OLTP side and I very rarely saw
|
||
applications like this.
|
||
|
||
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
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From pgsql-hackers-owner+M49465=pgman=candle.pha.pa.us@postgresql.org Wed Jan 28 13:30:48 2004
|
||
Return-path: <pgsql-hackers-owner+M49465=pgman=candle.pha.pa.us@postgresql.org>
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Wed, 28 Jan 2004 09:59:12 -0500 (EST)
|
||
To: Greg Stark <gsstark@mit.edu>
|
||
cc: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <871xpktb38.fsf@stark.xeocode.com>
|
||
References: <87isixt9h7.fsf@stark.xeocode.com> <29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com> <403.1075245881@sss.pgh.pa.us> <877jzcu85t.fsf@stark.xeocode.com> <5813.1075258139@sss.pgh.pa.us> <871xpktb38.fsf@stark.xeocode.com>
|
||
Comments: In-reply-to Greg Stark <gsstark@mit.edu>
|
||
message dated "28 Jan 2004 09:13:47 -0500"
|
||
Date: Wed, 28 Jan 2004 09:59:11 -0500
|
||
Message-ID: <18092.1075301951@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by amavisd-new at postgresql.org
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Status: OR
|
||
|
||
Greg Stark <gsstark@mit.edu> writes:
|
||
> Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
>> What sort?
|
||
|
||
> To build the in-memory bitmap you effectively have to do a sort.
|
||
|
||
Hm, you're thinking that the operation of inserting a bit into a bitmap
|
||
has to be at least O(log N). Seems to me that that depends on the data
|
||
structure you use. In principle it could be O(1), if you use a true
|
||
bitmap (linear array) -- just index and set the bit. You might be right
|
||
that practical data structures would be O(log N), but I'm not totally
|
||
convinced.
|
||
|
||
> If the tuples come out of the index in heap order then you can combine
|
||
> them without having to go through that step.
|
||
|
||
But considering the restrictions implied by that assumption --- no range
|
||
scans, no non-btree indexes --- I doubt we will take the trouble to
|
||
implement that variant. We'll want to do the generalized bitmap code
|
||
anyway.
|
||
|
||
In any case, this discussion is predicated on the assumption that the
|
||
operations involving the bitmap are a significant fraction of the total
|
||
time, which I think is quite uncertain. Until we build it and profile
|
||
it, we won't know that.
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 4: Don't 'kill -9' the postmaster
|
||
|
||
From pgsql-hackers-owner+M49457=pgman=candle.pha.pa.us@postgresql.org Wed Jan 28 10:42:58 2004
|
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Return-path: <pgsql-hackers-owner+M49457=pgman=candle.pha.pa.us@postgresql.org>
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
References: <87isixt9h7.fsf@stark.xeocode.com>
|
||
<29815.1075243916@sss.pgh.pa.us> <87d695t2ak.fsf@stark.xeocode.com>
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<403.1075245881@sss.pgh.pa.us> <877jzcu85t.fsf@stark.xeocode.com>
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<5813.1075258139@sss.pgh.pa.us> <871xpktb38.fsf@stark.xeocode.com>
|
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<18092.1075301951@sss.pgh.pa.us>
|
||
In-Reply-To: <18092.1075301951@sss.pgh.pa.us>
|
||
From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
|
||
Date: 28 Jan 2004 10:09:34 -0500
|
||
Message-ID: <87vfmwrtxt.fsf@stark.xeocode.com>
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version=2.61
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Status: ORr
|
||
|
||
|
||
Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
|
||
> In any case, this discussion is predicated on the assumption that the
|
||
> operations involving the bitmap are a significant fraction of the total
|
||
> time, which I think is quite uncertain. Until we build it and profile
|
||
> it, we won't know that.
|
||
|
||
The other thought I had was that it would be difficult to tell when to follow
|
||
this path. Since the main case where it wins is when the individual indexes
|
||
aren't very selective but the combination is very selective, and we don't have
|
||
inter-column correlation statistics ...
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 9: the planner will ignore your desire to choose an index scan if your
|
||
joining column's datatypes do not match
|
||
|
||
From pgsql-hackers-owner+M49467=pgman=candle.pha.pa.us@postgresql.org Wed Jan 28 17:29:11 2004
|
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Return-path: <pgsql-hackers-owner+M49467=pgman=candle.pha.pa.us@postgresql.org>
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Reply-To: <simon@2ndquadrant.com>
|
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From: "Simon Riggs" <simon@2ndquadrant.com>
|
||
To: "'Tom Lane'" <tgl@sss.pgh.pa.us>, "'Greg Stark'" <gsstark@mit.edu>
|
||
cc: <pgsql-hackers@postgresql.org>
|
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Subject: Re: [HACKERS] Question about indexes
|
||
Date: Wed, 28 Jan 2004 22:15:40 -0000
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version=2.61
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Status: OR
|
||
|
||
Some potentially helpful background comments on the discussion so far...
|
||
|
||
>Tom Lane writes
|
||
>>Greg Stark writes
|
||
>> Note that the space saving of bitmap indexes is still a substantial
|
||
>> factor.
|
||
>I think you are still confusing what I'm talking about with a bitmap
|
||
index, >ie, a persistent structure on-disk. It's not that at all, but a
|
||
transient >structure built in-memory during an index scan.
|
||
|
||
Oracle allows the creation of bitmap indices as persistent data
|
||
structures.
|
||
|
||
The "space saving" of bitmap indices is only a saving when compared with
|
||
btree indices. If you don't have them at all because they are built
|
||
dynamically when required, as Tom is suggesting, then you "save" even
|
||
more space.
|
||
|
||
Maintaining the bitmap index is a costly operation. You tend to want to
|
||
build them on "characteristic" columns, of which there tends to be more
|
||
of in a database than "partial/full identity" columns on which you build
|
||
btrees (forgive the vagueness of that comment), so you end up with loads
|
||
of the damn things, so the space soon adds up. It can be hard to judge
|
||
which ones are the important ones, especially when each is used by a
|
||
different user/group. Building them dynamically is a good way of solving
|
||
the question "which ones are needed?". Ever seen 58 indices on a table?
|
||
Don't go there.
|
||
|
||
My vote would be implement the dynamic building capability, then return
|
||
to implement a persisted structure later if that seems like it would be
|
||
a further improvement. [The option would be nice]
|
||
|
||
If we do it dynamically, as Tom suggests, then we don't have to code the
|
||
index maintenance logic at all and the functionality will be with us all
|
||
the sooner. Go Tom!
|
||
|
||
>Tom Lane writes
|
||
> In any case, this discussion is predicated on the assumption that the
|
||
> operations involving the bitmap are a significant fraction of the
|
||
total
|
||
> time, which I think is quite uncertain. Until we build it and profile
|
||
> it, we won't know that.
|
||
|
||
Dynamically building the bitmaps has been the strategy in use by
|
||
Teradata for nearly a decade on many large datawarehouses. I can
|
||
personally vouch for the effectiveness of this approach - I was
|
||
surprised when Oracle went for the persistent option. Certainly in that
|
||
case building the bitmaps adds much less time than is saved overall by
|
||
the better total query strategy.
|
||
|
||
>Greg Stark writes
|
||
> > To build the in-memory bitmap you effectively have to do a sort.
|
||
|
||
Not sure on this latter point: I think I agree with Greg on that point,
|
||
but want to believe Tom because requiring a sort will definitely add
|
||
time.
|
||
|
||
To shed some light in this area, some other major implementations are:
|
||
|
||
In Teradata, tables are stored based upon a primary index, which is
|
||
effectively an index-organised table. The index pointers are stored in
|
||
sorted order lock step with the blocks of the associated table - No sort
|
||
required. (The ordering is based upon a hashed index, but that doesn't
|
||
change the technique).
|
||
|
||
Oracle's tables/indexes use heaps/btrees also, though they do provide an
|
||
index-organised table feature similar to Teradata. Maybe the lack of
|
||
heap/btree consistent ordering in Oracle and their subsequent design
|
||
choice of persistent bitmap indices is an indication for PostgreSQL too?
|
||
|
||
In Oracle, bitmap indices are an important precursor to the star join
|
||
technique. AFAICS it is still possible to have a star join plan without
|
||
having persistent bitmap indices. IMHO, the longer term goal of a good
|
||
star join plan is an important one - that may influence the design
|
||
selection for this discussion.
|
||
|
||
Hope some of that helps,
|
||
|
||
Best regards, Simon Riggs
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 8: explain analyze is your friend
|
||
|
||
From pgsql-hackers-owner+M49477=pgman=candle.pha.pa.us@postgresql.org Thu Jan 29 04:24:47 2004
|
||
Return-path: <pgsql-hackers-owner+M49477=pgman=candle.pha.pa.us@postgresql.org>
|
||
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Subject: Re: [HACKERS] Question about indexes
|
||
Date: Thu, 29 Jan 2004 09:17:11 -0000
|
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Message-ID: <0A5B2E3C3A64CA4AB14F76DBCA76DDA44EF9B2@seifur.hnit.is>
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Thread-Topic: [HACKERS] Question about indexes
|
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Thread-Index: AcPl7J1SKohPpCtfSZq2EeeqhKLynAAW3BDw
|
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From: <lnd@hnit.is>
|
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To: <pgsql-hackers@postgresql.org>
|
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autolearn=no version=2.61
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Status: OR
|
||
|
||
|
||
A small comment on Oracle's implementation of persistent bitmap indexes:
|
||
|
||
Oracle's bitmap index is concurently locked by DML, i.e. it suites for OLAP
|
||
(basically read only data warehouses) but in no way for OLTP.
|
||
|
||
IMHO,
|
||
Laimis
|
||
|
||
> Maybe the lack of heap/btree consistent ordering in Oracle
|
||
> and their subsequent design choice of persistent bitmap
|
||
> indices is an indication for PostgreSQL too?
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 9: the planner will ignore your desire to choose an index scan if your
|
||
joining column's datatypes do not match
|
||
|
||
From pgsql-hackers-owner+M49497=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 01:22:15 2004
|
||
Return-path: <pgsql-hackers-owner+M49497=pgman=candle.pha.pa.us@postgresql.org>
|
||
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To: pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
References: <0A5B2E3C3A64CA4AB14F76DBCA76DDA44EF9B2@seifur.hnit.is>
|
||
In-Reply-To: <0A5B2E3C3A64CA4AB14F76DBCA76DDA44EF9B2@seifur.hnit.is>
|
||
From: Greg Stark <gsstark@mit.edu>
|
||
Organization: The Emacs Conspiracy; member since 1992
|
||
Date: 30 Jan 2004 01:16:21 -0500
|
||
Message-ID: <87y8rqx8p6.fsf@stark.xeocode.com>
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version=2.61
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||
Status: OR
|
||
|
||
|
||
<lnd@hnit.is> writes:
|
||
|
||
> A small comment on Oracle's implementation of persistent bitmap indexes:
|
||
>
|
||
> Oracle's bitmap index is concurently locked by DML, i.e. it suites for OLAP
|
||
> (basically read only data warehouses) but in no way for OLTP.
|
||
|
||
I knew this. I think they figured that was ok because bitmap indexes were
|
||
mainly intended to solve data warehouse problems anyways.
|
||
|
||
Thinking out loud here, I wonder whether this would be less of a problem for
|
||
postgres. Since tuples are never updated in place there would never be a need
|
||
to lock the entire bitmap until a transaction completes.
|
||
|
||
There would never be as much concurrency as btrees, assuming there was any
|
||
kind of compression on the bitmap, but I don't see any reason why a long-term
|
||
lock would have to be held for updates.
|
||
|
||
Even regular vacuum might not have to lock anything for long, just long enough
|
||
to clear the bits. and vacuum full/cluster already take table locks anyways.
|
||
|
||
I think the problem Oracle ran into was that storing rollback ids in the
|
||
bitmap is untenable. The whole point of persistent bitmap indexes is to store
|
||
a very dense representation that represents thousands of records per page.
|
||
Allocating space to store thousands of pending transaction ids and having
|
||
thousands of old versions of the page in the rollback segment would defeat the
|
||
purpose.
|
||
|
||
--
|
||
greg
|
||
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 7: don't forget to increase your free space map settings
|
||
|
||
From pgsql-hackers-owner+M49502=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 06:37:25 2004
|
||
Return-path: <pgsql-hackers-owner+M49502=pgman=candle.pha.pa.us@postgresql.org>
|
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by candle.pha.pa.us (8.11.6/8.11.6) id i0UBVHU04169;
|
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Fri, 30 Jan 2004 06:31:17 -0500 (EST)
|
||
From: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Message-ID: <200401301131.i0UBVHU04169@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <87vfmwrtxt.fsf@stark.xeocode.com>
|
||
To: Greg Stark <gsstark@mit.edu>
|
||
Date: Fri, 30 Jan 2004 06:31:17 -0500 (EST)
|
||
cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
|
||
X-Mailer: ELM [version 2.4ME+ PL108 (25)]
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Status: OR
|
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|
||
Greg Stark wrote:
|
||
>
|
||
> Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
>
|
||
> > In any case, this discussion is predicated on the assumption that the
|
||
> > operations involving the bitmap are a significant fraction of the total
|
||
> > time, which I think is quite uncertain. Until we build it and profile
|
||
> > it, we won't know that.
|
||
>
|
||
> The other thought I had was that it would be difficult to tell when to follow
|
||
> this path. Since the main case where it wins is when the individual indexes
|
||
> aren't very selective but the combination is very selective, and we don't have
|
||
> inter-column correlation statistics ...
|
||
|
||
I like the idea of building in-memory bitmapped indexes.
|
||
|
||
In your example, if you are restricting on A and B, and have no A,B
|
||
index but an A index and B index, why wouldn't you always create an
|
||
in-memory bitmapped index from indexes A and B, unless index A hits only
|
||
a few rows. In fact, from the optimizer statistics, you can guess on
|
||
how many bits you will hit from index A and index B, so we only have to
|
||
decide if it is better to take the more restrictive index and do heap
|
||
lookups for those, or scan the second index and then hit the heap. The
|
||
only thing A,B combined statistics would tell you is how many heap
|
||
matches you will find. The time to scan A and B indexes and create the
|
||
bitmap is already guessable from the single column statistics.
|
||
|
||
Also, what does an in-memory bitmapped index look like? Is it:
|
||
|
||
value: bitmap...
|
||
value: bitmap...
|
||
|
||
with the values organized in a btree fashion?
|
||
|
||
--
|
||
Bruce Momjian | http://candle.pha.pa.us
|
||
pgman@candle.pha.pa.us | (610) 359-1001
|
||
+ If your life is a hard drive, | 13 Roberts Road
|
||
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 6: Have you searched our list archives?
|
||
|
||
http://archives.postgresql.org
|
||
|
||
From pgsql-hackers-owner+M49505=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 09:55:27 2004
|
||
Return-path: <pgsql-hackers-owner+M49505=pgman=candle.pha.pa.us@postgresql.org>
|
||
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|
||
To: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <200401301131.i0UBVHU04169@candle.pha.pa.us>
|
||
References: <200401301131.i0UBVHU04169@candle.pha.pa.us>
|
||
Comments: In-reply-to Bruce Momjian <pgman@candle.pha.pa.us>
|
||
message dated "Fri, 30 Jan 2004 06:31:17 -0500"
|
||
Date: Fri, 30 Jan 2004 09:48:19 -0500
|
||
Message-ID: <12965.1075474099@sss.pgh.pa.us>
|
||
From: Tom Lane <tgl@sss.pgh.pa.us>
|
||
X-Virus-Scanned: by amavisd-new at postgresql.org
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|
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|
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|
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|
||
version=2.61
|
||
Status: ORr
|
||
|
||
Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> Also, what does an in-memory bitmapped index look like?
|
||
|
||
One idea that might work: a binary search tree in which each node
|
||
represents a single page of the table, and contains a bit array with
|
||
one bit for each possible item number on the page. You could not need
|
||
more than BLCKSZ/(sizeof(HeapTupleHeaderData)+sizeof(ItemIdData)) bits
|
||
in a node, or about 36 bytes at default BLCKSZ --- for most tables you
|
||
could probably prove it would be a great deal less. You only allocate
|
||
nodes for pages that have at least one interesting row.
|
||
|
||
I think this would represent a reasonable compromise between size and
|
||
insertion speed. It would only get large if the indexscan output
|
||
demanded visiting many different pages --- but at some point you could
|
||
abandon index usage and do a sequential scan, so I think that property
|
||
is okay.
|
||
|
||
A variant is to make the per-page bit arrays be entries in a hash table
|
||
with page number as hash key. This would reduce insertion to a nearly
|
||
constant-time operation, but the drawback is that you'd need an explicit
|
||
sort at the end to put the per-page entries into page number order
|
||
before you scan 'em. You might come out ahead anyway, not sure.
|
||
|
||
Or we could try a true linear bitmap (indexed by page number times
|
||
max-items-per-page plus item number) that's compressed in some fashion,
|
||
probably just by eliminating large runs of zeroes. The difficulty here
|
||
is that inserting a new one-bit could be pretty expensive, and we need
|
||
it to be cheap.
|
||
|
||
Perhaps someone can come up with other better ideas ...
|
||
|
||
regards, tom lane
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 8: explain analyze is your friend
|
||
|
||
From pgsql-hackers-owner+M49506=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 10:23:37 2004
|
||
Return-path: <pgsql-hackers-owner+M49506=pgman=candle.pha.pa.us@postgresql.org>
|
||
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|
||
by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id i0UFNZe17036
|
||
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|
||
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|
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|
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|
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|
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Fri, 30 Jan 2004 10:14:22 -0500 (EST)
|
||
From: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Message-ID: <200401301514.i0UFEMl15556@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <12965.1075474099@sss.pgh.pa.us>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
Date: Fri, 30 Jan 2004 10:14:22 -0500 (EST)
|
||
cc: Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
||
X-Mailer: ELM [version 2.4ME+ PL108 (25)]
|
||
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|
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|
||
Status: OR
|
||
|
||
Tom Lane wrote:
|
||
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> > Also, what does an in-memory bitmapped index look like?
|
||
>
|
||
> One idea that might work: a binary search tree in which each node
|
||
> represents a single page of the table, and contains a bit array with
|
||
> one bit for each possible item number on the page. You could not need
|
||
> more than BLCKSZ/(sizeof(HeapTupleHeaderData)+sizeof(ItemIdData)) bits
|
||
> in a node, or about 36 bytes at default BLCKSZ --- for most tables you
|
||
> could probably prove it would be a great deal less. You only allocate
|
||
> nodes for pages that have at least one interesting row.
|
||
|
||
Actually, I think I made a mistake. I was wondering what on-disk
|
||
bitmapped indexes look like.
|
||
|
||
--
|
||
Bruce Momjian | http://candle.pha.pa.us
|
||
pgman@candle.pha.pa.us | (610) 359-1001
|
||
+ If your life is a hard drive, | 13 Roberts Road
|
||
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
|
||
|
||
---------------------------(end of broadcast)---------------------------
|
||
TIP 9: the planner will ignore your desire to choose an index scan if your
|
||
joining column's datatypes do not match
|
||
|
||
From pgsql-hackers-owner+M49507=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 10:31:27 2004
|
||
Return-path: <pgsql-hackers-owner+M49507=pgman=candle.pha.pa.us@postgresql.org>
|
||
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|
||
by candle.pha.pa.us (8.11.6/8.11.6) with ESMTP id i0UFVOe18065
|
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|
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|
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|
||
Fri, 30 Jan 2004 10:22:34 -0500 (EST)
|
||
From: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Message-ID: <200401301522.i0UFMYr16926@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <87vfmwrtxt.fsf@stark.xeocode.com>
|
||
To: Greg Stark <gsstark@mit.edu>
|
||
Date: Fri, 30 Jan 2004 10:22:34 -0500 (EST)
|
||
cc: Tom Lane <tgl@sss.pgh.pa.us>, pgsql-hackers@postgresql.org
|
||
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|
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|
||
Greg Stark wrote:
|
||
>
|
||
> Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
>
|
||
> > In any case, this discussion is predicated on the assumption that the
|
||
> > operations involving the bitmap are a significant fraction of the total
|
||
> > time, which I think is quite uncertain. Until we build it and profile
|
||
> > it, we won't know that.
|
||
>
|
||
> The other thought I had was that it would be difficult to tell when to follow
|
||
> this path. Since the main case where it wins is when the individual indexes
|
||
> aren't very selective but the combination is very selective, and we don't have
|
||
> inter-column correlation statistics ...
|
||
|
||
We actually have heap access cost and index access cost. You could
|
||
compare costs of looking at all of index A's heap vs. looking at index
|
||
B and then hopefully fewer heap rows.
|
||
|
||
--
|
||
Bruce Momjian | http://candle.pha.pa.us
|
||
pgman@candle.pha.pa.us | (610) 359-1001
|
||
+ If your life is a hard drive, | 13 Roberts Road
|
||
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
|
||
|
||
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From alvherre@CM-lcon2-51-253.cm.vtr.net Fri Jan 30 10:24:32 2004
|
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Date: Fri, 30 Jan 2004 12:24:18 -0300
|
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From: Alvaro Herrera <alvherre@dcc.uchile.cl>
|
||
To: Tom Lane <tgl@sss.pgh.pa.us>
|
||
cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
|
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pgsql-hackers@postgresql.org
|
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Subject: Re: [HACKERS] Question about indexes
|
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Message-ID: <20040130152418.GB24123@dcc.uchile.cl>
|
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us> <12965.1075474099@sss.pgh.pa.us>
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Status: ORr
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On Fri, Jan 30, 2004 at 09:48:19AM -0500, Tom Lane wrote:
|
||
|
||
> A variant is to make the per-page bit arrays be entries in a hash table
|
||
> with page number as hash key. This would reduce insertion to a nearly
|
||
> constant-time operation, but the drawback is that you'd need an explicit
|
||
> sort at the end to put the per-page entries into page number order
|
||
> before you scan 'em. You might come out ahead anyway, not sure.
|
||
|
||
Is there a reason sort the pages before scanning them? The result won't
|
||
come out sorted one way or the other.
|
||
|
||
--
|
||
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
|
||
"Para tener m<>s hay que desear menos"
|
||
|
||
From pgsql-hackers-owner+M49508=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 10:33:18 2004
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cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] Question about indexes
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us> <12965.1075474099@sss.pgh.pa.us>
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On Fri, Jan 30, 2004 at 09:48:19AM -0500, Tom Lane wrote:
|
||
|
||
> A variant is to make the per-page bit arrays be entries in a hash table
|
||
> with page number as hash key. This would reduce insertion to a nearly
|
||
> constant-time operation, but the drawback is that you'd need an explicit
|
||
> sort at the end to put the per-page entries into page number order
|
||
> before you scan 'em. You might come out ahead anyway, not sure.
|
||
|
||
Is there a reason sort the pages before scanning them? The result won't
|
||
come out sorted one way or the other.
|
||
|
||
--
|
||
Alvaro Herrera (<alvherre[a]dcc.uchile.cl>)
|
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"Para tener m<>s hay que desear menos"
|
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---------------------------(end of broadcast)---------------------------
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From pgsql-hackers-owner+M49509=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 10:39:11 2004
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From: Bruce Momjian <pgman@candle.pha.pa.us>
|
||
Message-ID: <200401301530.i0UFUgQ18014@candle.pha.pa.us>
|
||
Subject: Re: [HACKERS] Question about indexes
|
||
In-Reply-To: <20040130152418.GB24123@dcc.uchile.cl>
|
||
To: Alvaro Herrera <alvherre@dcc.uchile.cl>
|
||
Date: Fri, 30 Jan 2004 10:30:42 -0500 (EST)
|
||
cc: Tom Lane <tgl@sss.pgh.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
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|
||
Alvaro Herrera wrote:
|
||
> On Fri, Jan 30, 2004 at 09:48:19AM -0500, Tom Lane wrote:
|
||
>
|
||
> > A variant is to make the per-page bit arrays be entries in a hash table
|
||
> > with page number as hash key. This would reduce insertion to a nearly
|
||
> > constant-time operation, but the drawback is that you'd need an explicit
|
||
> > sort at the end to put the per-page entries into page number order
|
||
> > before you scan 'em. You might come out ahead anyway, not sure.
|
||
>
|
||
> Is there a reason sort the pages before scanning them? The result won't
|
||
> come out sorted one way or the other.
|
||
|
||
I think the goal would be to hit the heap in sequential order as much as
|
||
possible. When we are doing reading right from the index, we haven't
|
||
collected all the heap values in one place, but since we have them in
|
||
memory, we might as well sort them, though I don't think that is a
|
||
requirement, just a performance enhancement, or at least that is my
|
||
guess.
|
||
|
||
--
|
||
Bruce Momjian | http://candle.pha.pa.us
|
||
pgman@candle.pha.pa.us | (610) 359-1001
|
||
+ If your life is a hard drive, | 13 Roberts Road
|
||
+ Christ can be your backup. | Newtown Square, Pennsylvania 19073
|
||
|
||
---------------------------(end of broadcast)---------------------------
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||
TIP 8: explain analyze is your friend
|
||
|
||
From hannu@tm.ee Fri Jan 30 17:44:13 2004
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Subject: Re: [HACKERS] Question about indexes
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|
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
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cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
|
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<12965.1075474099@sss.pgh.pa.us>
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version=2.61
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Status: OR
|
||
|
||
Tom Lane kirjutas R, 30.01.2004 kell 16:48:
|
||
> Bruce Momjian <pgman@candle.pha.pa.us> writes:
|
||
> > Also, what does an in-memory bitmapped index look like?
|
||
>
|
||
> One idea that might work: a binary search tree in which each node
|
||
> represents a single page of the table, and contains a bit array with
|
||
> one bit for each possible item number on the page. You could not need
|
||
> more than BLCKSZ/(sizeof(HeapTupleHeaderData)+sizeof(ItemIdData)) bits
|
||
> in a node, or about 36 bytes at default BLCKSZ --- for most tables you
|
||
> could probably prove it would be a great deal less. You only allocate
|
||
> nodes for pages that have at least one interesting row.
|
||
|
||
Another idea would be using bitmaps where we have just one bit per
|
||
database page and do a seq scan but just over marked pages.
|
||
|
||
Even when allocating them in full such indexes would occupy just
|
||
1/(8k*8bit) of the amount they describe, so index for 1GB table would be
|
||
1G/(8k*8bit) = 16 kilobytes (2 pages)
|
||
|
||
Also, such indexes, if persistent, could also be used (together with
|
||
FSM) when deciding placement of new tuples, so they provide a form of
|
||
clustering.
|
||
|
||
This would of course be most useful for data-warehouse type operations,
|
||
where database is significantöy bigger than memory.
|
||
|
||
And the seqscan over bitmap should not be done in simple page order, but
|
||
rather in two passes -
|
||
1. over those pages which are already in cache (either postgresqls
|
||
or systems (if we find a way to get such info from the system))
|
||
2. in sequential order over the rest.
|
||
|
||
> I think this would represent a reasonable compromise between size and
|
||
> insertion speed. It would only get large if the indexscan output
|
||
> demanded visiting many different pages --- but at some point you could
|
||
> abandon index usage and do a sequential scan, so I think that property
|
||
> is okay.
|
||
|
||
One case where almost full intermediate bitmap could be needed is when
|
||
doing a star join or just AND of several conditions, where each single
|
||
index spans a significant part of the table, but the result does not.
|
||
|
||
> A variant is to make the per-page bit arrays be entries in a hash table
|
||
> with page number as hash key. This would reduce insertion to a nearly
|
||
> constant-time operation, but the drawback is that you'd need an explicit
|
||
> sort at the end to put the per-page entries into page number order
|
||
> before you scan 'em. You might come out ahead anyway, not sure.
|
||
>
|
||
> Or we could try a true linear bitmap (indexed by page number times
|
||
> max-items-per-page plus item number) that's compressed in some fashion,
|
||
> probably just by eliminating large runs of zeroes. The difficulty here
|
||
> is that inserting a new one-bit could be pretty expensive, and we need
|
||
> it to be cheap.
|
||
>
|
||
> Perhaps someone can come up with other better ideas ...
|
||
|
||
I have also contemplated a scenario, where we could use some
|
||
not-quite-max power-of-2 bits-per-page linear bitmap and mark intra-page
|
||
wraps (when we tried to mark a point past that not-quite-max number in a
|
||
page) in high bit (or another bitmap) making info for that page folded.
|
||
AN example would be setting bit 40 in 32-bits/page index - this would
|
||
set bit 40&31 and mark the page folded.
|
||
|
||
When combining such indexes using AND or OR, we need some spcial
|
||
handling of folded pages, but could still get non-folded (0) results out
|
||
from AND of 2 folded pages if the bits are distributed nicely.
|
||
|
||
--------------
|
||
Hannu
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
|
||
From pgsql-hackers-owner+M49529=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 18:10:22 2004
|
||
Return-path: <pgsql-hackers-owner+M49529=pgman=candle.pha.pa.us@postgresql.org>
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Fri, 30 Jan 2004 18:02:58 -0500 (EST)
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To: Hannu Krosing <hannu@tm.ee>
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cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] Question about indexes
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In-Reply-To: <1075502634.4007.32.camel@fuji.krosing.net>
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us> <12965.1075474099@sss.pgh.pa.us> <1075502634.4007.32.camel@fuji.krosing.net>
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Comments: In-reply-to Hannu Krosing <hannu@tm.ee>
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message dated "Sat, 31 Jan 2004 00:43:54 +0200"
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||
Date: Fri, 30 Jan 2004 18:02:58 -0500
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Message-ID: <20776.1075503778@sss.pgh.pa.us>
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From: Tom Lane <tgl@sss.pgh.pa.us>
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Status: OR
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Hannu Krosing <hannu@tm.ee> writes:
|
||
> Another idea would be using bitmaps where we have just one bit per
|
||
> database page and do a seq scan but just over marked pages.
|
||
|
||
That seems a bit too lossy for me, but I really like your later idea
|
||
about folding. Generalizing that a little, we can choose any fold point
|
||
we like. We could allocate, say, one 32-bit word per page and set the
|
||
(i mod 32) bit when item i is fingered by the index. After retrieving
|
||
the heap page, we'd need to test all the valid rows that have item
|
||
numbers matching a set bit mod 32. On typical tables (with circa 100
|
||
items per page) this would require testing only about 3 rows per page.
|
||
ORing and ANDing of such bitmaps still works, with the understanding
|
||
that it's lossy and you have to double check each retrieved tuple.
|
||
|
||
If the fold point is above about 100, your idea of keeping track of
|
||
whether we actually set any wrapped-around bits would become useful,
|
||
but below that I think we'd just be wasting a bit.
|
||
|
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regards, tom lane
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From tgl@sss.pgh.pa.us Fri Jan 30 18:03:08 2004
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To: Hannu Krosing <hannu@tm.ee>
|
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cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
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Subject: Re: [HACKERS] Question about indexes
|
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In-Reply-To: <1075502634.4007.32.camel@fuji.krosing.net>
|
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us> <12965.1075474099@sss.pgh.pa.us> <1075502634.4007.32.camel@fuji.krosing.net>
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Comments: In-reply-to Hannu Krosing <hannu@tm.ee>
|
||
message dated "Sat, 31 Jan 2004 00:43:54 +0200"
|
||
Date: Fri, 30 Jan 2004 18:02:58 -0500
|
||
Message-ID: <20776.1075503778@sss.pgh.pa.us>
|
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From: Tom Lane <tgl@sss.pgh.pa.us>
|
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Status: OR
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Hannu Krosing <hannu@tm.ee> writes:
|
||
> Another idea would be using bitmaps where we have just one bit per
|
||
> database page and do a seq scan but just over marked pages.
|
||
|
||
That seems a bit too lossy for me, but I really like your later idea
|
||
about folding. Generalizing that a little, we can choose any fold point
|
||
we like. We could allocate, say, one 32-bit word per page and set the
|
||
(i mod 32) bit when item i is fingered by the index. After retrieving
|
||
the heap page, we'd need to test all the valid rows that have item
|
||
numbers matching a set bit mod 32. On typical tables (with circa 100
|
||
items per page) this would require testing only about 3 rows per page.
|
||
ORing and ANDing of such bitmaps still works, with the understanding
|
||
that it's lossy and you have to double check each retrieved tuple.
|
||
|
||
If the fold point is above about 100, your idea of keeping track of
|
||
whether we actually set any wrapped-around bits would become useful,
|
||
but below that I think we'd just be wasting a bit.
|
||
|
||
regards, tom lane
|
||
|
||
From hannu@tm.ee Fri Jan 30 18:21:59 2004
|
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Subject: Re: [HACKERS] Question about indexes
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From: Hannu Krosing <hannu@tm.ee>
|
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To: Tom Lane <tgl@sss.pgh.pa.us>
|
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cc: Bruce Momjian <pgman@candle.pha.pa.us>, Greg Stark <gsstark@mit.edu>,
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pgsql-hackers@postgresql.org
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In-Reply-To: <20776.1075503778@sss.pgh.pa.us>
|
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us>
|
||
<12965.1075474099@sss.pgh.pa.us>
|
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<1075502634.4007.32.camel@fuji.krosing.net>
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<20776.1075503778@sss.pgh.pa.us>
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Tom Lane kirjutas L, 31.01.2004 kell 01:02:
|
||
> Hannu Krosing <hannu@tm.ee> writes:
|
||
> > Another idea would be using bitmaps where we have just one bit per
|
||
> > database page and do a seq scan but just over marked pages.
|
||
>
|
||
> That seems a bit too lossy for me,
|
||
|
||
I originally thought of it in context of data-warehousing and persistent
|
||
bitmap indexes. there the use of these same bitmaps for clustering would
|
||
un-lossify this approach.
|
||
|
||
> but I really like your later idea
|
||
> about folding. Generalizing that a little, we can choose any fold point
|
||
> we like. We could allocate, say, one 32-bit word per page and set the
|
||
> (i mod 32) bit when item i is fingered by the index. After retrieving
|
||
> the heap page, we'd need to test all the valid rows that have item
|
||
> numbers matching a set bit mod 32. On typical tables (with circa 100
|
||
> items per page) this would require testing only about 3 rows per page.
|
||
> ORing and ANDing of such bitmaps still works, with the understanding
|
||
> that it's lossy and you have to double check each retrieved tuple.
|
||
>
|
||
> If the fold point is above about 100, your idea of keeping track of
|
||
> whether we actually set any wrapped-around bits would become useful,
|
||
> but below that I think we'd just be wasting a bit.
|
||
|
||
Not only wasting bits, but also making the code hairier - we can't just
|
||
do simple ANDs and ORs.
|
||
|
||
--------------
|
||
Hannu
|
||
|
||
From gsstark@mit.edu Fri Jan 30 19:04:21 2004
|
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|
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Greg Stark <gsstark@mit.edu>, pgsql-hackers@postgresql.org
|
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Subject: Re: [HACKERS] Question about indexes
|
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us>
|
||
<12965.1075474099@sss.pgh.pa.us>
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<1075502634.4007.32.camel@fuji.krosing.net>
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<20776.1075503778@sss.pgh.pa.us>
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In-Reply-To: <20776.1075503778@sss.pgh.pa.us>
|
||
From: Greg Stark <gsstark@mit.edu>
|
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Organization: The Emacs Conspiracy; member since 1992
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Date: 30 Jan 2004 19:04:03 -0500
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|
||
|
||
Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
|
||
> That seems a bit too lossy for me, but I really like your later idea
|
||
> about folding. Generalizing that a little, we can choose any fold point
|
||
> we like. We could allocate, say, one 32-bit word per page and set the
|
||
> (i mod 32) bit when item i is fingered by the index. After retrieving
|
||
> the heap page, we'd need to test all the valid rows that have item
|
||
> numbers matching a set bit mod 32. On typical tables (with circa 100
|
||
> items per page) this would require testing only about 3 rows per page.
|
||
> ORing and ANDing of such bitmaps still works, with the understanding
|
||
> that it's lossy and you have to double check each retrieved tuple.
|
||
|
||
That would make it really hard to ever clear the bits. What do you do when you
|
||
vacuum and one of the tuples is no longer needed. You can't be sure you can
|
||
clear the bit in the index because there could be multiple tuples represented
|
||
by the bit being set. You would have to test the condition on the other tuples
|
||
covered by the bit to see if it can be cleared.
|
||
|
||
--
|
||
greg
|
||
|
||
From pgsql-hackers-owner+M49533=pgman=candle.pha.pa.us@postgresql.org Fri Jan 30 19:56:45 2004
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Return-path: <pgsql-hackers-owner+M49533=pgman=candle.pha.pa.us@postgresql.org>
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To: Greg Stark <gsstark@mit.edu>
|
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cc: Hannu Krosing <hannu@tm.ee>, Bruce Momjian <pgman@candle.pha.pa.us>,
|
||
pgsql-hackers@postgresql.org
|
||
Subject: Re: [HACKERS] Question about indexes
|
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In-Reply-To: <87wu79vv9o.fsf@stark.xeocode.com>
|
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References: <200401301131.i0UBVHU04169@candle.pha.pa.us> <12965.1075474099@sss.pgh.pa.us> <1075502634.4007.32.camel@fuji.krosing.net> <20776.1075503778@sss.pgh.pa.us> <87wu79vv9o.fsf@stark.xeocode.com>
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Comments: In-reply-to Greg Stark <gsstark@mit.edu>
|
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message dated "30 Jan 2004 19:04:03 -0500"
|
||
Date: Fri, 30 Jan 2004 19:50:23 -0500
|
||
Message-ID: <23292.1075510223@sss.pgh.pa.us>
|
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From: Tom Lane <tgl@sss.pgh.pa.us>
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X-Virus-Scanned: by amavisd-new at postgresql.org
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Status: OR
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Greg Stark <gsstark@mit.edu> writes:
|
||
> Tom Lane <tgl@sss.pgh.pa.us> writes:
|
||
>> ORing and ANDing of such bitmaps still works, with the understanding
|
||
>> that it's lossy and you have to double check each retrieved tuple.
|
||
|
||
> That would make it really hard to ever clear the bits.
|
||
|
||
We're speaking of in-memory bitmaps constructed on-the-fly here. You're
|
||
right that it wouldn't work for persistent indexes, but I'm not very
|
||
interested in that case at the moment ...
|
||
|
||
regards, tom lane
|
||
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---------------------------(end of broadcast)---------------------------
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TIP 8: explain analyze is your friend
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